feat(p2p-16): add labeled read-source telemetry (#671)

[jlon]

Summary

• Add labeled observability for read-source attribution, fallback reasons, and P2P metrics.
• Keep protocol and product-path behavior unchanged; this PR is telemetry-only.

Design

• Extract tenant and job labels at the read boundary and keep label policy setup centralized.
• Isolate telemetry plumbing from filesystem API churn and unrelated cleanup.

Implementation

• Add labeled metrics in client_metrics and thread read-source and fallback telemetry through block_reader.
• Wire tenant/job label extraction and snapshot sync through fs_reader_base and fs_context.

Verification

• Run labeled read-source and fallback metrics tests.
• Run one cached-read regression test plus cargo test -p curvine-client --lib --no-run.

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