fix(core): Remove outdated `_experiments.enableMetrics` references from metrics JSDoc

[github-actions]

Note

The pull request "fix(core): Remove outdated _experiments.enableMetrics references from metrics JSDoc" was created by @chargome but did not reference an issue. Therefore this issue was created for better visibility in external tools like Linear.

Metrics are enabled by default and no longer require the _experiments.enableMetrics flag. This removes the outdated JSDoc @summary references in count(), gauge(), and distribution() that still mentioned needing to enable the experimental option.

[linear]

JS-1673

[github-actions]

A PR closing this issue has just been released 🚀

This issue was referenced by PR #19252, which was included in the 10.39.0 release.