monitoring: add repo cnt to dashboard (#48653)

it will be nice to show repo count in the dashboard

context:
https://sourcegraph.slack.com/archives/C03LCPCT3SP/p1677875955414389

cloud would like to compare repo count across all instances

## Test plan

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CI

doc/admin/observability/dashboards.md

@@ -4992,6 +4992,29 @@ Query: `sum by (category)(increase(src_frontend_internal_request_duration_second
 
 <br />
 
+#### gitserver: src_gitserver_repo_count
+
+<p class="subtitle">Number of repositories on gitserver</p>
+
+This metric is only for informational purposes. It indicates the total number of repositories on gitserver.
+
+It does not indicate any problems with the instance.
+
+This panel has no related alerts.
+
+To see this panel, visit `/-/debug/grafana/d/gitserver/gitserver?viewPanel=100090` on your Sourcegraph instance.
+
+<sub>*Managed by the [Sourcegraph Repo Management team](https://handbook.sourcegraph.com/departments/engineering/teams/repo-management).*</sub>
+
+<details>
+<summary>Technical details</summary>
+
+Query: `src_gitserver_repo_count`
+
+</details>
+
+<br />
+
 ### Git Server: Gitserver: Gitserver API (powered by internal/observation)
 
 #### gitserver: gitserver_api_total

monitoring/definitions/git_server.go

@@ -275,6 +275,22 @@ func GitServer() *monitoring.Dashboard {
 						},
 						shared.FrontendInternalAPIErrorResponses("gitserver", monitoring.ObservableOwnerRepoManagement).Observable(),
 					},
+					{
+						{
+							Name:          "src_gitserver_repo_count",
+							Description:   "number of repositories on gitserver",
+							Query:         "src_gitserver_repo_count",
+							NoAlert:       true,
+							Panel:         monitoring.Panel().LegendFormat("repo count"),
+							Owner:         monitoring.ObservableOwnerRepoManagement,
+							MultiInstance: true,
+							Interpretation: `
+								This metric is only for informational purposes. It indicates the total number of repositories on gitserver.
+
+								It does not indicate any problems with the instance.
+							`,
+						},
+					},
 				},
 			},
 			shared.GitServer.NewAPIGroup(containerName),