feat(metrique-util): prototype request-scoped metrics pools

[rcoh]

Summary

• implement MetricsPool and cloneable MetricsPoolHandle for heterogeneous child metrics flattened into a parent entry
• add explicit-handle and poll-scoped current-pool APIs with runtime casing, prefixes, close semantics, and rate-limited last-write-wins collision handling
• add an AWS SDK interceptor example with scoped collection, explicit injection, and global fallback
• add focused tests and a Divan benchmark for pool sizes 1, 4, and 16

Stack

This draft implementation is stacked on RFC PR #357. Its diff contains only the prototype; retarget it to main after #357 merges.

Testing

• cargo +nightly fmt --all
• cargo clippy --workspace --all-features -- -D warnings
• cargo +1.91 nextest run -p metrique-util --test metrics_pool (7 passed)
• cargo +1.91 test -p metrique-util --example sdk-interceptor-metrics-pool --features metrics-pool (3 passed)
• CARGO_INCREMENTAL=0 cargo +1.91 nextest run (697 passed)
• cargo bench -p metrique-util --bench metrics_pool --features metrics-pool