[receiver/prometheus] fix panic on shutdown (#16470)

Author: povilasv

.chloggen/prometheusreceiver-shutdown.yaml

@@ -0,0 +1,16 @@
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: bug_fix
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: prometheusreceiver
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: "Fix prometheus receiver panic on shutdown"
+
+# One or more tracking issues related to the change
+issues: [16469]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:

receiver/prometheusreceiver/metrics_receiver.go

@@ -312,8 +312,12 @@ func gcInterval(cfg *config.Config) time.Duration {
 
 // Shutdown stops and cancels the underlying Prometheus scrapers.
 func (r *pReceiver) Shutdown(context.Context) error {
-	r.cancelFunc()
-	r.scrapeManager.Stop()
+	if r.cancelFunc != nil {
+		r.cancelFunc()
+	}
+	if r.scrapeManager != nil {
+		r.scrapeManager.Stop()
+	}
 	close(r.targetAllocatorStop)
 	return nil
 }