Plugins: Make backend plugin metrics endpoints available with optional authentication (#46467)

* add new endpoint without auth+config

* add cfg check

* fit lint issue

* Add basic auth support

Co-authored-by: Will Browne <wbrowne@users.noreply.github.com>

* WIP docs

* Update docs/sources/administration/view-server/internal-metrics.md

Co-authored-by: Dave Henderson <dhenderson@gmail.com>

* update instructions

Co-authored-by: Will Browne <will.browne@grafana.com>
Co-authored-by: Will Browne <wbrowne@users.noreply.github.com>
Co-authored-by: Dave Henderson <dhenderson@gmail.com>

pkg/api/http_server.go

@@ -497,6 +497,7 @@ func (hs *HTTPServer) addMiddlewaresAndStaticRoutes() {
 	m.Use(hs.healthzHandler)
 	m.Use(hs.apiHealthHandler)
 	m.Use(hs.metricsEndpoint)
+	m.Use(hs.pluginMetricsEndpoint)
 
 	m.Use(hs.ContextHandler.Middleware)
 	m.Use(middleware.OrgRedirect(hs.Cfg))