[chore]: Bump github.com/prometheus/client_golang from 1.20.5 to 1.21.0 (#3860)

Bumps [github.com/prometheus/client_golang](https://github.com/prometheus/client_golang) from 1.20.5 to 1.21.0.
- [Release notes](https://github.com/prometheus/client_golang/releases)
- [Changelog](https://github.com/prometheus/client_golang/blob/main/CHANGELOG.md)
- [Commits](https://github.com/prometheus/client_golang/compare/v1.20.5...v1.21.0)

---
updated-dependencies:
- dependency-name: github.com/prometheus/client_golang
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

vendor/github.com/prometheus/common/expfmt/openmetrics_create.go

@@ -38,7 +38,7 @@ type EncoderOption func(*encoderOption)
 
 // WithCreatedLines is an EncoderOption that configures the OpenMetrics encoder
 // to include _created lines (See
-// https://github.com/OpenObservability/OpenMetrics/blob/main/specification/OpenMetrics.md#counter-1).
+// https://github.com/prometheus/OpenMetrics/blob/v1.0.0/specification/OpenMetrics.md#counter-1).
 // Created timestamps can improve the accuracy of series reset detection, but
 // come with a bandwidth cost.
 //
@@ -102,7 +102,7 @@ func WithUnit() EncoderOption {
 //
 //   - According to the OM specs, the `# UNIT` line is optional, but if populated,
 //     the unit has to be present in the metric name as its suffix:
-//     (see https://github.com/OpenObservability/OpenMetrics/blob/main/specification/OpenMetrics.md#unit).
+//     (see https://github.com/prometheus/OpenMetrics/blob/v1.0.0/specification/OpenMetrics.md#unit).
 //     However, in order to accommodate any potential scenario where such a change in the
 //     metric name is not desirable, the users are here given the choice of either explicitly
 //     opt in, in case they wish for the unit to be included in the output AND in the metric name