Add byte-level metrics for create/update/delete payload operations

[Copilot]

The store already counted create/update/delete operations, but it did not expose the corresponding data volume. This change adds byte-based metrics so writes and removals can be observed in terms of payload size, not just operation count.

• Metrics added

  • Registers three new counters:
    • arkiv_store/creates_bytes
    • arkiv_store/updates_bytes
    • arkiv_store/deletes_bytes

• Accounting changes

  • Extends per-block stats to accumulate payload bytes alongside existing operation counts.
  • Records bytes from the payload involved in each operation:
    • create: len(operation.Create.Content)
    • update: len(operation.Update.Content)
    • delete/expire: len(latestPayload.Payload) for the payload actually removed

• Batch reporting

  • Includes create/update/delete byte totals in batch-level logging so operational logs match the new counters.

• Coverage

  • Adds focused test coverage for byte counters across create → update → delete flow, asserting counter deltas match the payload sizes processed.

metricCreatesBytes = metrics.NewRegisteredCounter("arkiv_store/creates_bytes", nil)
metricUpdatesBytes = metrics.NewRegisteredCounter("arkiv_store/updates_bytes", nil)
metricDeletesBytes = metrics.NewRegisteredCounter("arkiv_store/deletes_bytes", nil)

blockStat.createsBytes += len(operation.Create.Content)
blockStat.updatesBytes += len(operation.Update.Content)
blockStat.deletesBytes += len(latestPayload.Payload)

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