Add metrics collector unit tests

[BennettSchwartz]

Summary

pkg/metrics/metrics.go computes several aggregate metrics but currently has no unit test coverage.

Why this is a good first issue

• Good exposure to record schema and observability outputs.
• Can be tested with a lightweight fake store.
• No API contract changes required.

Proposed work

Add pkg/metrics/metrics_test.go that validates:

• Basic aggregation (TotalRecords, RecordsByType, averages, salience buckets).
• RFC 15.10 derived fields (MemoryGrowthRate, RetrievalUsefulness, RevisionRate, etc.).
• Empty-record behavior.
• Error path when store List fails.

Acceptance criteria

• Tests pass with go test ./pkg/metrics.
• Snapshot fields are asserted with deterministic fixtures.
• Both success and failure paths are covered.