`--metrics` does not expose span histogram metrics unless `--tracing` is also enabled

[brg8]

Summary

The --metrics flag only exposes a small subset of metrics unless --tracing is also turned on. In our production deployment, with metrics enabled, we only see:

• rollup_boost_execution_mode
• rollup_boost_rpc_blocks_created

The docs suggest that enabling --metrics should also expose span duration histograms such as:

• fork_choice_updated_v3_duration
• get_payload_v3_duration
• new_payload_v3_duration

However, those histogram metrics appear to be gated behind --tracing, not --metrics (code ref).

Suggested fix

I think the cleanest fix would be to decouple Prometheus span metrics from OTLP tracing export.

[0xForerunner]

Some of the metrics are co-generated with the traces. So you need tracing to enable metrics. I recommend enabling --tracing and just ignoring the traces. We could disable the otlp export when we're missing the tracing flag but I'm not 100% sure that's worth the hassle.