Add comments, prettify tests.

internal/component/loki/process/process_test.go

@@ -515,14 +515,15 @@ func TestMetricsStageRefresh(t *testing.T) {
 	loki_process_custom_paulin_test{filename="/var/log/pods/agent/agent/1.log",foo="bar"} %d
 	`
 
+	// The component will be reconfigured so that it has a metric.
 	t.Run("config with a metric", func(t *testing.T) {
 		tester.updateAndTest(numLogsToSend, cfgWithMetric,
 			"",
 			fmt.Sprintf(cfgWithMetric_Metrics, numLogsToSend))
 	})
 
 	// The component will be "updated" with the same config.
-	// We expect the metric to stay the same, because the component should be smart enough to
+	// We expect the metric to stay the same before logs are sent - the component should be smart enough to
 	// know that the new config is the same as the old one and it should just keep running as it is.
 	// If it resets the metric, this could cause issues with some users who have a sidecar "autoreloader"
 	// which reloads the collector config every X seconds.
@@ -536,8 +537,9 @@ func TestMetricsStageRefresh(t *testing.T) {
 	// Use a config which has no metrics stage.
 	// This should cause the metric to disappear.
 	cfgWithNoStages := forwardArgs
-
-	tester.updateAndTest(numLogsToSend, cfgWithNoStages, "", "")
+	t.Run("config with no metrics stage", func(t *testing.T) {
+		tester.updateAndTest(numLogsToSend, cfgWithNoStages, "", "")
+	})
 
 	// Use a config which has a metric with a different name,
 	// as well as a metric with the same name as the one in the previous config.
@@ -566,9 +568,11 @@ func TestMetricsStageRefresh(t *testing.T) {
 	loki_process_custom_paulin_test{filename="/var/log/pods/agent/agent/1.log",foo="bar"} %d
 	`
 
-	tester.updateAndTest(numLogsToSend, cfgWithTwoMetrics,
-		"",
-		fmt.Sprintf(expectedMetrics3, numLogsToSend, numLogsToSend))
+	t.Run("config with a new and old metric", func(t *testing.T) {
+		tester.updateAndTest(numLogsToSend, cfgWithTwoMetrics,
+			"",
+			fmt.Sprintf(expectedMetrics3, numLogsToSend, numLogsToSend))
+	})
 }
 
 type tester struct {
@@ -645,7 +649,7 @@ func (t *tester) updateAndTest(numLogsToSend int, cfg, expectedMetricsBeforeSend
 	// Check the component metrics.
 	if err := testutil.GatherAndCompare(t.registry,
 		strings.NewReader(expectedMetricsBeforeSendingLogs)); err != nil {
-		t.t.Fatalf("mismatch metrics: %v", err)
+		require.NoError(t.t, err)
 	}
 
 	// Send logs.
@@ -668,6 +672,6 @@ func (t *tester) updateAndTest(numLogsToSend int, cfg, expectedMetricsBeforeSend
 	// Check the component metrics.
 	if err := testutil.GatherAndCompare(t.registry,
 		strings.NewReader(expectedMetricsAfterSendingLogs)); err != nil {
-		t.t.Fatalf("mismatch metrics: %v", err)
+		require.NoError(t.t, err)
 	}
 }

internal/util/unregisterer.go

@@ -18,6 +18,22 @@ func WrapWithUnregisterer(reg prometheus.Registerer) *Unregisterer {
 	}
 }
 
+// An "unchecked collector" is a collector which returns an empty description.
+// It is described in the Prometheus documentation, here:
+// https://pkg.go.dev/github.com/prometheus/client_golang/prometheus#hdr-Custom_Collectors_and_constant_Metrics
+//
+// > Alternatively, you could return no Desc at all, which will mark the Collector “unchecked”.
+// > No checks are performed at registration time, but metric consistency will still be ensured at scrape time,
+// > i.e. any inconsistencies will lead to scrape errors. Thus, with unchecked Collectors,
+// > the responsibility to not collect metrics that lead to inconsistencies in the total scrape result
+// > lies with the implementer of the Collector. While this is not a desirable state, it is sometimes necessary.
+// > The typical use case is a situation where the exact metrics to be returned by a Collector cannot be predicted
+// > at registration time, but the implementer has sufficient knowledge of the whole system to guarantee metric consistency.
+//
+// Unchecked collectors are used in the Loki "metrics" stage of the Loki "process" component.
+//
+// The isUncheckedCollector function is similar to how Prometheus' Go client extracts the metric description:
+// https://github.com/prometheus/client_golang/blob/45f1e72421d9d11af6be784ad60b7389f7543e70/prometheus/registry.go#L372-L381
 func isUncheckedCollector(c prometheus.Collector) bool {
 	descChan := make(chan *prometheus.Desc, 10)