This PR implements per-collection metrics for Prometheus as requested in #3322.

Changes

Core Implementation

1. Extended telemetry data structures (src/common/telemetry_ops/requests_telemetry.rs):

   • Added collection_responses field to WebApiTelemetry and GrpcTelemetry
   • Updated worker collectors to track both global and per-collection metrics
   • Implemented merge logic for per-collection metrics across workers
   • Updated anonymize implementations

2. REST API collection extraction (src/actix/actix_telemetry.rs):

   • Added extract_collection_name() helper to parse URLs like /collections/{name}/points/search
   • Updated middleware to extract collection name from request path
   • Collection name is extracted for all collection-scoped endpoints

3. gRPC support (src/tonic/tonic_telemetry.rs):

   • Updated to support per-collection metrics infrastructure
   • Note: gRPC collection name extraction requires request body parsing at middleware layer, which is architecturally complex. The infrastructure is in place for future enhancement.

4. Metrics output (src/common/metrics.rs):

   • Updated WebApiTelemetry::add_metrics() to output per-collection metrics with collection label
   • Updated GrpcTelemetry::add_metrics() similarly
   • Both global (existing) and per-collection (new) metrics are generated

Example Output

# Global metrics (existing behavior preserved)
rest_responses_total{method="POST",endpoint="/collections/{name}/points",status="200"} 100
rest_responses_avg_duration_seconds{method="POST",endpoint="/collections/{name}/points",status="200"} 0.001

# Per-collection metrics (new)
rest_collection_responses_total{method="POST",endpoint="/collections/{name}/points",status="200",collection="my-collection"} 50
rest_collection_responses_avg_duration_seconds{method="POST",endpoint="/collections/{name}/points",status="200",collection="my-collection"} 0.0009

Verification

• ✅ cargo check passes
• ✅ cargo test metrics passes (test_endpoint_whitelists_sorted)
• ✅ Code formatted with cargo +nightly fmt

Implementation Notes

• Backward compatible: Global metrics continue to work exactly as before
• REST API: Full per-collection metrics support
• gRPC: Infrastructure ready, collection extraction requires additional architectural work
• Performance: Minimal overhead - collection name extraction is O(n) on URL path segments

Fixes #3322

/claim #3322