Deprecate metrics for WAF

Why are we doing this work

More information about deprecation: #271276 (closed)

We have to deprecate metrics that will no longer be used in GitLab.

Relevant links

• Epic
• Metrics deprecation process

Non-functional requirements

• [-] Documentation:
• [-] Feature flag:
• [-] Performance:
• [-] Testing:

Implementation plan

• backend update related Modsecurity metrics to status: removed according to process described here: #284637 (comment 449786696)
  • config/metrics/counts_all/20210216175442_ingress_modsecurity_packets_processed.yml
  • config/metrics/counts_all/20210216175444_ingress_modsecurity_packets_anomalous.yml
  • config/metrics/counts_all/20210216175450_ingress_modsecurity_logging.yml
  • config/metrics/counts_all/20210216175452_ingress_modsecurity_blocking.yml
  • config/metrics/counts_all/20210216175454_ingress_modsecurity_disabled.yml
  • config/metrics/counts_all/20210216175456_ingress_modsecurity_not_installed.yml
  • config/metrics/settings/20210216175459_ingress_modsecurity_enabled.yml
  • ee/config/metrics/counts_all/20210216175441_ingress_modsecurity_statistics_unavailable.yml
  • remove related metrics from event dictionary: doc/development/usage_ping/dictionary.md