monitoring: add more panels to Grafana repo-updater dashboard

Created by: unknwon

HEADS UP I'm on vacation since Oct 1st, anyone is free to take over this PR if it's not able to get merged before then.

List of repo syncing related metrics

Checked off indicates it has been utilized for at least one panel.

repo-updater (/-/debug/proxies/repo-updater-127.0.0.1/metrics):

• src_repoupdater_purge_failed
• src_repoupdater_purge_success
• src_repoupdater_sched_auto_fetch
• src_repoupdater_sched_manual_fetch
• src_repoupdater_sched_error
• src_repoupdater_sched_known_repos
• src_repoupdater_sched_loops
• src_repoupdater_source_duration_*
• src_repoupdater_source_repos_total
• src_repoupdater_store_done_*
• src_repoupdater_store_list_external_repo_specs_*
• src_repoupdater_store_list_external_services_*
• src_repoupdater_store_list_repos_*
• src_repoupdater_store_set_cloned_repos_*
• src_repoupdater_store_transact_*
• src_repoupdater_store_upsert_repos_*
• src_repoupdater_store_upsert_sources_*
• src_repoupdater_syncer_sync_duration_*
• src_repoupdater_syncer_sync_last_time
• src_repoupdater_syncer_synced_repos_total{state=*}
• src_repoupdater_external_services_total
• src_repoupdater_completed_sync_jobs_total
• src_repoupdater_errored_sync_jobs_total
• src_repoupdater_sched_update_queue_length

gitserver: no metrics to answer the question of Number of update requests to gitservers.

TODO

• Decide if store_* metrics are useful in general to be added as panels in Grafana dashboard. Ryan: I don't think we need these yet
• Decide if new Prometheus metrics are needed.

Fixes #14237