Resolve "Failure in `ee/spec/features/markdown/metrics_spec.rb:52`"

What does this MR do?

Fix master:broken where ee/lib/ee/banzai/pipeline/gfm_pipeline.rb has multiple definitions of .metrics_filters.

The latter overrode [::Banzai::Filter::InlineAlertMetricsFilter, *super] with [::Banzai::Filter::InlineClusterMetricsFilter, *super]

Closes #207931 (closed)