[ dart:developer ] Update documentation for `Metrics`
[ dart:developer ] Update documentation for `Metrics`

Fixes https://github.com/dart-lang/sdk/issues/49511

Change-Id: I0dae4709ef34d1dd9cf3c8ccebd295df4565fbc3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/252620
Commit-Queue: Ben Konyi <67ffcaedcc3dc3d87dddb9627b72dd41756b31b1@google.com>
Reviewed-by: Devon Carew <e996497f4f2225f0ee7a9f4d1696ee710b488b66@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk