Metrics: Delete GFM pipeline logic for embeds

Overview

⚠ Removal should take place during %16.0 or later.

⚠ Category:Metrics code is intertwined with the metrics GitLab reports on itself. Only Category:Metrics should be removed. Similarly, some items named like PrometheusAlert are also used for AlertManagement::Alert and should not be removed.

As part of Monitor: Metrics Deprecation and Removal (&10107 - closed), we want to remove the GitLab-Flavored Markdown Prometheus & Grafana metrics embeds portion of Category:Metrics.

Notable details

• groupobservability has added their own support for GFM embeds. This should not be removed.
• The order in which frontend & backend remove metrics embed code doesn't matter. Nothing will appear physically broken.
• We don't need to worry about invalidating the cache for markdown fields. After the frontend pieces are removed, there will be zero difference in how markdown containing a metrics dashboard url is presented.

Implementation plan

1. Remove metrics-related banzai filters
2. Remove metrics-related banzai redactors
3. Cleanup class inheritance in observability embed banzai filter

Relevant code

The lists below are meant as an aid & may not be exhaustive.

Deletions:

• lib/banzai/filter/inline_alert_metrics_filter.rb
• lib/banzai/filter/inline_cluster_metrics_filter.rb
• lib/banzai/filter/inline_metrics_filter.rb
• lib/banzai/filter/inline_grafana_metrics_filter.rb
• lib/banzai/filter/inline_metrics_redactor_filter.rb
• spec/features/markdown/metrics_spec.rb
• ee/spec/features/markdown/metrics_spec.rb

Cleanups:

• lib/banzai/pipeline/gfm_pipeline.rb
• lib/banzai/filter/inline_embeds_filter.rb (still partially required for observability embeds)
• ee/lib/ee/banzai/pipeline/gfm_pipeline.rb