CLIENT-5630 Support metrics sampling and exporters

Summary

Implements the shared client metrics v3 collection/snapshot model for the Go client (CLIENT-5630), aligned with the collection + exporter tech specs.

• Flags / tiers: master (metrics.enabled) with independent operational and usage; DefaultMetricsPolicy / nil / zero-value EnableMetrics → master + operational (legacy); YAML v3 enabled without extended.operational leaves operational off.
• Sampler: one metrics_on per user API call (range/threshold, math/rand/v2); gates command-path latency, bytes, error splits, command.count / retries.
• Dual latency (Decision 15): cluster.latency.{read,write,batch,query} (whole successful call); namespace.latency.* (successful node RPC/sub-batch); node.latency.conn (successful open, not sampler-gated). New types/histogram/latency_buckets.go for the shared columns+shift layout; existing SyncHistogram kept for Go fine-grained command/phase metrics.
• CM-7 / Decision 14: client vs server timeouts via fromServer; namespace error/timeout/key_busy/record_too_big/device_overload on each failed attempt; bytes on RPC success only.
• CM-8: node connection.{open,tls,auth}.failure vs cluster validation.*; auth only on auth result codes (network/TLS during login → open/tls); open.failure rolls up TLS/auth.
• Usage (CM-17 / CM-27): feature.api.* plus shapes and filter.ael|exp (including per-record batch filters); no object-mapping (N/A) and no CM-28.
• Batch / query / scan: parent owns command.count + cluster latency; children record namespace latency only; batch.partial_failure once per key on successful batch completion; one-key batches stay on the wire path when metrics are on.
• Export: NONE | FILE | CUSTOM; new snake_case file exporter (header(3)); nodes_departed cap 256, cleared only after a fully successful periodic export; on-demand includes but does not drain. Dynconfig re-apply is idempotent (no export-ticker restart on unchanged policy). Separate on-demand vs export CPU sample windows.
• Also: dynconfig static hydrate no longer wipes ClientPolicy credentials; Connection.updateDeadline no longer extends a sub-ms remaining TotalTimeout past the absolute deadline.