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/backendplugin/collector/collector.go

@@ -0,0 +1,89 @@
+package collector
+
+import (
+	"context"
+	"sync"
+	"time"
+
+	"github.com/grafana/grafana/pkg/infra/log"
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+// Namespace collector metric namespace
+const Namespace = "grafana_plugin"
+
+var (
+	scrapeDurationDesc = prometheus.NewDesc(
+		prometheus.BuildFQName(Namespace, "scrape", "duration_seconds"),
+		"grafana_plugin: Duration of a plugin collector scrape.",
+		[]string{"plugin_id"},
+		nil,
+	)
+	scrapeSuccessDesc = prometheus.NewDesc(
+		prometheus.BuildFQName(Namespace, "scrape", "success"),
+		"grafana_plugin: Whether a plugin collector succeeded.",
+		[]string{"plugin_id"},
+		nil,
+	)
+)
+
+// Collector is the interface a plugin collector has to implement.
+type Collector interface {
+	// Get new metrics and expose them via prometheus registry.
+	CollectMetrics(ctx context.Context, ch chan<- prometheus.Metric) error
+}
+
+// PluginCollector implements the prometheus.Collector interface.
+type PluginCollector struct {
+	collectors map[string]Collector
+	logger     log.Logger
+}
+
+// NewPluginCollector creates a new PluginCollector..
+func NewPluginCollector() PluginCollector {
+	return PluginCollector{
+		collectors: make(map[string]Collector),
+		logger:     log.New("plugins.backend.collector"),
+	}
+}
+
+func (pc PluginCollector) Register(pluginID string, c Collector) {
+	pc.collectors[pluginID] = c
+}
+
+// Describe implements the prometheus.Collector interface.
+func (pc PluginCollector) Describe(ch chan<- *prometheus.Desc) {
+	ch <- scrapeDurationDesc
+	ch <- scrapeSuccessDesc
+}
+
+// Collect implements the prometheus.Collector interface.
+func (pc PluginCollector) Collect(ch chan<- prometheus.Metric) {
+	ctx := context.Background()
+	wg := sync.WaitGroup{}
+	wg.Add(len(pc.collectors))
+	for name, c := range pc.collectors {
+		go func(name string, c Collector) {
+			execute(ctx, name, c, ch, pc.logger)
+			wg.Done()
+		}(name, c)
+	}
+	wg.Wait()
+}
+
+func execute(ctx context.Context, pluginID string, c Collector, ch chan<- prometheus.Metric, logger log.Logger) {
+	begin := time.Now()
+	err := c.CollectMetrics(ctx, ch)
+	duration := time.Since(begin)
+	var success float64
+
+	if err != nil {
+		logger.Error("collector failed", "pluginId", pluginID, "took", duration, "error", err)
+		success = 0
+	} else {
+		logger.Debug("collector succeeded", "pluginId", pluginID, "took", duration)
+		success = 1
+	}
+	ch <- prometheus.MustNewConstMetric(scrapeDurationDesc, prometheus.GaugeValue, duration.Seconds(), pluginID)
+	ch <- prometheus.MustNewConstMetric(scrapeSuccessDesc, prometheus.GaugeValue, success, pluginID)
+}