Backend Plugins: Collect and expose metrics and plugin process health check (#21481)

Adds support for collecting metrics from backend plugins and 
exposing them thru Grafana's Prometheus metrics endpoint. 
Enables to check health of backend plugin by using the route
`/api/plugins/<plugin id>/health`.
Uses sdk v0.6.0.

Closes #20984

pkg/plugins/plugins.go

@@ -14,6 +14,7 @@ import (
 	"time"
 
 	"github.com/grafana/grafana/pkg/infra/log"
+	"github.com/grafana/grafana/pkg/infra/metrics"
 	"github.com/grafana/grafana/pkg/plugins/backendplugin"
 	"github.com/grafana/grafana/pkg/registry"
 	"github.com/grafana/grafana/pkg/setting"
@@ -110,6 +111,12 @@ func (pm *PluginManager) Init() error {
 		app.initApp()
 	}
 
+	for _, p := range Plugins {
+		if !p.IsCorePlugin {
+			metrics.SetPluginBuildInformation(p.Id, p.Type, p.Info.Version)
+		}
+	}
+
 	return nil
 }