reset the basemap when the labelsbuilder is reset (#11771)

This PR fixes an issue where the `label_format` stage sometimes doesn't
populate labels when it should.

The label `baz` exists on all log lines but is only populated on the
first line.

![image](https://github.com/grafana/loki/assets/1413241/86585a94-937f-4e07-9657-b94699ce8586)

The issue is this call to
[IntoMap](https://github.com/grafana/loki/blob/06bb20914b027c96ac3af27ae36e91f35529524d/pkg/logql/log/fmt.go#L403).
It tries to avoid copying the map multiple times
[here](https://github.com/grafana/loki/blob/06bb20914b027c96ac3af27ae36e91f35529524d/pkg/logql/log/labels.go#L482)
but the `baseMap` variable isn't reset when the labels builder is reset.
As a result, calls to to `IntoMap` after the builder has been reset mean
that `!b.hasDel() && !b.hasAdd() && !b.HasErr()` evaluates to `true` but
`b.baseMap == nil` also evaluates to `true` so no items are copied.

This PR makes it so that `b.baseMap` is reset along with the rest of the
builder's state in `Reset`

pkg/logql/log/labels.go

@@ -135,6 +135,7 @@ type BaseLabelsBuilder struct {
 	errDetails string
 
 	groups                       []string
+	baseMap                      map[string]string
 	parserKeyHints               ParserHint // label key hints for metric queries that allows to limit parser extractions to only this list of labels.
 	without, noLabels            bool
 	referencedStructuredMetadata bool
@@ -146,7 +147,6 @@ type BaseLabelsBuilder struct {
 // LabelsBuilder is the same as labels.Builder but tailored for this package.
 type LabelsBuilder struct {
 	base          labels.Labels
-	baseMap       map[string]string
 	buf           labels.Labels
 	currentResult LabelsResult
 	groupedResult LabelsResult
@@ -211,6 +211,7 @@ func (b *BaseLabelsBuilder) Reset() {
 	}
 	b.err = ""
 	b.errDetails = ""
+	b.baseMap = nil
 	b.parserKeyHints.Reset()
 }
 
@@ -481,9 +482,9 @@ func (b *LabelsBuilder) IntoMap(m map[string]string) {
 	if !b.hasDel() && !b.hasAdd() && !b.HasErr() {
 		if b.baseMap == nil {
 			b.baseMap = b.base.Map()
-			for k, v := range b.baseMap {
-				m[k] = v
-			}
+		}
+		for k, v := range b.baseMap {
+			m[k] = v
 		}
 		return
 	}

pkg/logql/log/labels_test.go

@@ -68,6 +68,64 @@ func TestLabelsBuilder_LabelsError(t *testing.T) {
 	require.Equal(t, labels.FromStrings("already", "in"), lbs)
 }
 
+func TestLabelsBuilder_IntoMap(t *testing.T) {
+	strs := []string{
+		"namespace", "loki",
+		"job", "us-central1/loki",
+		"cluster", "us-central1",
+		"ToReplace", "text",
+	}
+	lbs := labels.FromStrings(strs...)
+
+	t.Run("it still copies the map after a Reset", func(t *testing.T) {
+		b := NewBaseLabelsBuilder().ForLabels(lbs, lbs.Hash())
+
+		m := map[string]string{}
+		b.IntoMap(m)
+
+		require.Equal(t, map[string]string{
+			"namespace": "loki",
+			"job":       "us-central1/loki",
+			"cluster":   "us-central1",
+			"ToReplace": "text",
+		}, m)
+
+		b.Reset()
+
+		m2 := map[string]string{}
+		b.IntoMap(m2)
+		require.Equal(t, map[string]string{
+			"namespace": "loki",
+			"job":       "us-central1/loki",
+			"cluster":   "us-central1",
+			"ToReplace": "text",
+		}, m2)
+	})
+
+	t.Run("it can copy the map several times", func(t *testing.T) {
+		b := NewBaseLabelsBuilder().ForLabels(lbs, lbs.Hash())
+
+		m := map[string]string{}
+		b.IntoMap(m)
+
+		require.Equal(t, map[string]string{
+			"namespace": "loki",
+			"job":       "us-central1/loki",
+			"cluster":   "us-central1",
+			"ToReplace": "text",
+		}, m)
+
+		m2 := map[string]string{}
+		b.IntoMap(m2)
+		require.Equal(t, map[string]string{
+			"namespace": "loki",
+			"job":       "us-central1/loki",
+			"cluster":   "us-central1",
+			"ToReplace": "text",
+		}, m2)
+	})
+}
+
 func TestLabelsBuilder_LabelsResult(t *testing.T) {
 	strs := []string{
 		"namespace", "loki",