Convert some metrics to Histograms (#50420)

Because Summary metrics can not be aggreated, convert them to histograms
so that users with HA deployments can use these metrics.
* Convert metrics registration to promauto.
* Improve help text style.

Signed-off-by: SuperQ <superq@gmail.com>

pkg/plugins/backendplugin/instrumentation/instrumentation.go

@@ -5,29 +5,23 @@ import (
 	"time"
 
 	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
 )
 
 var (
-	pluginRequestCounter  *prometheus.CounterVec
-	pluginRequestDuration *prometheus.SummaryVec
-)
-
-func init() {
-	pluginRequestCounter = prometheus.NewCounterVec(prometheus.CounterOpts{
+	pluginRequestCounter = promauto.NewCounterVec(prometheus.CounterOpts{
 		Namespace: "grafana",
 		Name:      "plugin_request_total",
 		Help:      "The total amount of plugin requests",
 	}, []string{"plugin_id", "endpoint", "status"})
 
-	pluginRequestDuration = prometheus.NewSummaryVec(prometheus.SummaryOpts{
-		Namespace:  "grafana",
-		Name:       "plugin_request_duration_milliseconds",
-		Help:       "Plugin request duration",
-		Objectives: map[float64]float64{0.5: 0.05, 0.9: 0.01, 0.99: 0.001},
+	pluginRequestDuration = promauto.NewHistogramVec(prometheus.HistogramOpts{
+		Namespace: "grafana",
+		Name:      "plugin_request_duration_milliseconds",
+		Help:      "Plugin request duration",
+		Buckets:   []float64{.005, .01, .025, .05, .1, .25, .5, 1, 2.5, 5, 10, 25, 50, 100},
 	}, []string{"plugin_id", "endpoint"})
-
-	prometheus.MustRegister(pluginRequestCounter, pluginRequestDuration)
-}
+)
 
 // instrumentPluginRequest instruments success rate and latency of `fn`
 func instrumentPluginRequest(pluginID string, endpoint string, fn func() error) error {