fix(datadog encoder): remove false-positive compression estimator check that panics on first write

[thieman]

Problem

When a pathological metric (e.g. a sketch with many bins generated by an extremely low sample rate) was the very first input encoded into a request builder, the agent would panic:

builder told us to flush, but gave us nothing

Root cause: CompressionEstimator::would_write_exceed_threshold had an early exit:

if len > threshold {
    return true;
}

If the metric's uncompressed encoded size exceeded the configured compressed size limit, this guard fired and returned true — signalling that a flush was needed — even though nothing had been written to the compressor yet. encode_inner returned Ok(false) (flush signal), flush() returned an empty vec, and the caller panicked.

The check is logically wrong: uncompressed size being larger than the compressed limit says nothing about whether the compressed output will fit. Sketches with thousands of near-identical bins (the exact pathological case) compress extremely well. The estimator's purpose is to be an optimization hint, not a hard gate.

Fix

Remove the early len > threshold exit from would_write_exceed_threshold. The existing total_compressed_len == 0 guard (which already returns false when no data has been written) is sufficient — and is now the first thing evaluated.

If the payload genuinely is too large after compression, the backstop is flush() at the compressed_len > compressed_limit check: for a single oversized metric it clears the input and returns Err(PayloadTooLarge), which the encoder caller logs and drops rather than panicking.

This matches the approach @tobz suggested in #1305.

Test plan

• first_input_larger_than_compressed_limit_does_not_panic — creates a builder with a compressed limit (1 KB) smaller than the input (10 KB of repeated chars), encodes the large input as the very first metric, and asserts no panic
• Updated compression_estimator_no_output unit test — now asserts false for both cases when no compressed data has been written, reflecting the new semantics
• All existing request builder and compression estimator tests pass

Closes #1119. See also #1305.

🤖 Generated with Claude Code

[pr-commenter]

Binary Size Analysis (Agent Data Plane)

Target: b7aedc3 (baseline) vs 4651645 (comparison) diff
Analysis Type: Stripped binaries (debug symbols excluded)
Baseline Size: 26.35 MiB
Comparison Size: 26.35 MiB
Size Change: +0 B (+0.00%)
Pass/Fail Threshold: +5%
Result: PASSED ✅

Changes by Module

Module	File Size	Symbols
[Unmapped]	+188 B	1
saluki_components::common::datadog	-174 B	6
[sections]	-14 B	2

Detailed Symbol Changes

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +4.1%    +188  [ = ]       0    [Unmapped]
  +0.0%      +4  +0.0%      +4    [section .gcc_except_table]
  -0.1%     -14  -0.1%     -14    saluki_components::common::datadog::request_builder::RequestBuilder<E>::encode_inner::_{{closure}}::h9e42713475fa50b4
  -0.4%     -16  -0.4%     -16    saluki_components::common::datadog::request_builder::RequestBuilder<E>::encode_inner::_{{closure}}::h6e9889bd0f773199
  -0.0%     -18  -0.0%     -18    [section .text]
  -0.3%     -27  -0.3%     -27    saluki_components::common::datadog::request_builder::RequestBuilder<E>::encode_inner::_{{closure}}::hf5a628a3a0b06c9a
  -0.2%     -30  -0.2%     -30    saluki_components::common::datadog::request_builder::RequestBuilder<E>::encode_inner::_{{closure}}::h9eedd2bd0e10ef6a
  -0.2%     -30  -0.2%     -30    saluki_components::common::datadog::request_builder::RequestBuilder<E>::encode_inner::_{{closure}}::hcd0b9ca9428d1499
  -0.9%     -57  -0.9%     -57    saluki_components::common::datadog::request_builder::RequestBuilder<E>::encode_inner::_{{closure}}::h329de940b9ccc923
  [ = ]       0  -0.0%    -188    TOTAL

[pr-commenter]

Regression Detector (Agent Data Plane)

Regression Detector Results

Run ID: d2fe7750-b3bd-4c4b-8fb3-30552e6fbedc

Baseline: 592597c
Comparison: 3c85d43
Diff

Optimization Goals: ✅ No significant changes detected

Experiments ignored for regressions

Regressions in experiments with settings containing erratic: true are ignored.

perf	experiment	goal	Δ mean %	Δ mean % CI	trials	links
❌	otlp_ingest_logs_5mb_memory	memory utilization	+7.43	[+6.89, +7.96]	1	(metrics) (profiles) (logs)
➖	otlp_ingest_logs_5mb_cpu	% cpu utilization	+0.44	[-4.71, +5.58]	1	(metrics) (profiles) (logs)
➖	otlp_ingest_logs_5mb_throughput	ingress throughput	-0.02	[-0.14, +0.10]	1	(metrics) (profiles) (logs)

Fine details of change detection per experiment

perf	experiment	goal	Δ mean %	Δ mean % CI	trials	links
➖	dsd_uds_512kb_3k_contexts_cpu	% cpu utilization	+21.05	[-40.86, +82.97]	1	(metrics) (profiles) (logs)
➖	dsd_uds_1mb_3k_contexts_cpu	% cpu utilization	+14.40	[-43.18, +71.98]	1	(metrics) (profiles) (logs)
➖	dsd_uds_10mb_3k_contexts_cpu	% cpu utilization	+11.71	[-18.41, +41.84]	1	(metrics) (profiles) (logs)
➖	otlp_ingest_metrics_5mb_cpu	% cpu utilization	+7.44	[-1.29, +16.18]	1	(metrics) (profiles) (logs)
❌	otlp_ingest_logs_5mb_memory	memory utilization	+7.43	[+6.89, +7.96]	1	(metrics) (profiles) (logs)
➖	otlp_ingest_traces_5mb_cpu	% cpu utilization	+2.79	[+0.77, +4.80]	1	(metrics) (profiles) (logs)
➖	dsd_uds_500mb_3k_contexts_throughput	ingress throughput	+0.72	[+0.59, +0.85]	1	(metrics) (profiles) (logs)
➖	dsd_uds_500mb_3k_contexts_cpu	% cpu utilization	+0.47	[-0.95, +1.90]	1	(metrics) (profiles) (logs)
➖	otlp_ingest_logs_5mb_cpu	% cpu utilization	+0.44	[-4.71, +5.58]	1	(metrics) (profiles) (logs)
➖	otlp_ingest_traces_5mb_memory	memory utilization	+0.43	[+0.18, +0.68]	1	(metrics) (profiles) (logs)
➖	quality_gates_rss_dsd_medium	memory utilization	+0.41	[+0.21, +0.60]	1	(metrics) (profiles) (logs)
➖	dsd_uds_100mb_3k_contexts_memory	memory utilization	+0.38	[+0.20, +0.57]	1	(metrics) (profiles) (logs)
➖	quality_gates_rss_idle	memory utilization	+0.30	[+0.26, +0.35]	1	(metrics) (profiles) (logs)
➖	dsd_uds_10mb_3k_contexts_memory	memory utilization	+0.18	[-0.01, +0.36]	1	(metrics) (profiles) (logs)
➖	quality_gates_rss_dsd_low	memory utilization	+0.15	[-0.05, +0.34]	1	(metrics) (profiles) (logs)
➖	dsd_uds_1mb_3k_contexts_memory	memory utilization	+0.13	[-0.04, +0.31]	1	(metrics) (profiles) (logs)
➖	otlp_ingest_traces_ottl_filtering_5mb_memory	memory utilization	+0.07	[-0.27, +0.41]	1	(metrics) (profiles) (logs)
➖	quality_gates_rss_dsd_ultraheavy	memory utilization	+0.05	[-0.08, +0.18]	1	(metrics) (profiles) (logs)
➖	quality_gates_rss_dsd_heavy	memory utilization	+0.03	[-0.11, +0.17]	1	(metrics) (profiles) (logs)
➖	dsd_uds_100mb_3k_contexts_throughput	ingress throughput	+0.01	[-0.02, +0.04]	1	(metrics) (profiles) (logs)
➖	dsd_uds_512kb_3k_contexts_throughput	ingress throughput	+0.00	[-0.05, +0.05]	1	(metrics) (profiles) (logs)
➖	dsd_uds_1mb_3k_contexts_throughput	ingress throughput	+0.00	[-0.05, +0.06]	1	(metrics) (profiles) (logs)
➖	otlp_ingest_traces_5mb_throughput	ingress throughput	+0.00	[-0.02, +0.02]	1	(metrics) (profiles) (logs)
➖	otlp_ingest_traces_ottl_transform_5mb_memory	memory utilization	+0.00	[-0.24, +0.24]	1	(metrics) (profiles) (logs)
➖	otlp_ingest_traces_ottl_filtering_5mb_throughput	ingress throughput	-0.00	[-0.02, +0.02]	1	(metrics) (profiles) (logs)
➖	dsd_uds_10mb_3k_contexts_throughput	ingress throughput	-0.00	[-0.14, +0.13]	1	(metrics) (profiles) (logs)
➖	otlp_ingest_traces_ottl_transform_5mb_throughput	ingress throughput	-0.00	[-0.02, +0.02]	1	(metrics) (profiles) (logs)
➖	otlp_ingest_metrics_5mb_throughput	ingress throughput	-0.01	[-0.14, +0.12]	1	(metrics) (profiles) (logs)
➖	otlp_ingest_logs_5mb_throughput	ingress throughput	-0.02	[-0.14, +0.10]	1	(metrics) (profiles) (logs)
➖	dsd_uds_512kb_3k_contexts_memory	memory utilization	-0.26	[-0.45, -0.08]	1	(metrics) (profiles) (logs)
➖	dsd_uds_500mb_3k_contexts_memory	memory utilization	-0.37	[-0.53, -0.20]	1	(metrics) (profiles) (logs)
➖	otlp_ingest_traces_ottl_filtering_5mb_cpu	% cpu utilization	-0.92	[-3.28, +1.44]	1	(metrics) (profiles) (logs)
➖	otlp_ingest_traces_ottl_transform_5mb_cpu	% cpu utilization	-1.94	[-4.11, +0.24]	1	(metrics) (profiles) (logs)
➖	otlp_ingest_metrics_5mb_memory	memory utilization	-2.19	[-2.35, -2.02]	1	(metrics) (profiles) (logs)
➖	dsd_uds_100mb_3k_contexts_cpu	% cpu utilization	-3.55	[-9.28, +2.19]	1	(metrics) (profiles) (logs)

Bounds Checks: ✅ Passed

perf	experiment	bounds_check_name	replicates_passed	observed_value	links
✅	quality_gates_rss_dsd_heavy	memory_usage	10/10	111.74MiB ≤ 140MiB	(metrics) (profiles) (logs)
✅	quality_gates_rss_dsd_low	memory_usage	10/10	34.25MiB ≤ 50MiB	(metrics) (profiles) (logs)
✅	quality_gates_rss_dsd_medium	memory_usage	10/10	53.11MiB ≤ 75MiB	(metrics) (profiles) (logs)
✅	quality_gates_rss_dsd_ultraheavy	memory_usage	10/10	169.16MiB ≤ 200MiB	(metrics) (profiles) (logs)
✅	quality_gates_rss_idle	memory_usage	10/10	20.90MiB ≤ 40MiB	(metrics) (profiles) (logs)

Explanation

Confidence level: 90.00%
Effect size tolerance: |Δ mean %| ≥ 5.00%

Performance changes are noted in the perf column of each table:

• ✅ = significantly better comparison variant performance
• ❌ = significantly worse comparison variant performance
• ➖ = no significant change in performance

A regression test is an A/B test of target performance in a repeatable rig, where "performance" is measured as "comparison variant minus baseline variant" for an optimization goal (e.g., ingress throughput). Due to intrinsic variability in measuring that goal, we can only estimate its mean value for each experiment; we report uncertainty in that value as a 90.00% confidence interval denoted "Δ mean % CI".

For each experiment, we decide whether a change in performance is a "regression" -- a change worth investigating further -- if all of the following criteria are true:

1. Its estimated |Δ mean %| ≥ 5.00%, indicating the change is big enough to merit a closer look.

2. Its 90.00% confidence interval "Δ mean % CI" does not contain zero, indicating that if our statistical model is accurate, there is at least a 90.00% chance there is a difference in performance between baseline and comparison variants.

3. Its configuration does not mark it "erratic".