Adds per-collection request telemetry for REST and gRPC, exposed as Prometheus metrics with collection label. Bounds cardinality via LRU (QDRANT_TELEMETRY_COLLECTION_LIMIT, default 1000). Keeps existing global metrics unchanged. Extracts collection from REST path with URL decoding. Includes tests/per_collection_metrics_test.sh. /claim #3322 fixes #3322