lambda-promtail: cloudwatch: add '__aws_log_type' label (grafana#11335)

**What this PR does / why we need it**:

Adds the `__aws_log_type` label to AWS CloudWatch logs in
`lambda-promtail`.

**Which issue(s) this PR fixes**:

N/A

**Special notes for your reviewer**:

AWS S3 & Kinesis log types already have this label. The
`lambda-promtail` documentation
[here](https://github.com/grafana/loki/blob/main/docs/sources/send-data/lambda-promtail/_index.md?plain=1#L154)
suggests that CloudWatch logs has this label added as well, but in
practice it does not AFAICT.

**Checklist**
- [x] Reviewed the
[`CONTRIBUTING.md`](https://github.com/grafana/loki/blob/main/CONTRIBUTING.md)
guide (**required**)
- [ ] Documentation added
- [x] Tests updated
- [ ] `CHANGELOG.md` updated
- [ ] If the change is worth mentioning in the release notes, add
`add-to-release-notes` label
- [ ] Changes that require user attention or interaction to upgrade are
documented in `docs/sources/setup/upgrade/_index.md`
- [ ] For Helm chart changes bump the Helm chart version in
`production/helm/loki/Chart.yaml` and update
`production/helm/loki/CHANGELOG.md` and
`production/helm/loki/README.md`. [Example
PR](grafana@d10549e)
- [ ] If the change is deprecating or removing a configuration option,
update the `deprecated-config.yaml` and `deleted-config.yaml` files
respectively in the `tools/deprecated-config-checker` directory.
[Example
PR](grafana@0d4416a)

tools/lambda-promtail/lambda-promtail/cw.go

@@ -18,6 +18,7 @@ func parseCWEvent(ctx context.Context, b *batch, ev *events.CloudwatchLogsEvent)
 	}
 
 	labels := model.LabelSet{
+		model.LabelName("__aws_log_type"):             model.LabelValue("cloudwatch"),
 		model.LabelName("__aws_cloudwatch_log_group"): model.LabelValue(data.LogGroup),
 		model.LabelName("__aws_cloudwatch_owner"):     model.LabelValue(data.Owner),
 	}

tools/lambda-promtail/lambda-promtail/cw_test.go

@@ -0,0 +1,60 @@
+package main
+
+import (
+	"context"
+	"testing"
+
+	"github.com/aws/aws-lambda-go/events"
+	"github.com/stretchr/testify/require"
+
+	"github.com/grafana/loki/pkg/logproto"
+)
+
+func Test_parseCWEvent(t *testing.T) {
+	tests := []struct {
+		name           string
+		b              *batch
+		expectedStream string
+		keepStream     bool
+	}{
+		{
+			name: "cloudwatch",
+			b: &batch{
+				streams: map[string]*logproto.Stream{},
+			},
+			expectedStream: `{__aws_cloudwatch_log_group="testLogGroup", __aws_cloudwatch_owner="123456789123", __aws_log_type="cloudwatch"}`,
+			keepStream:     false,
+		},
+		{
+			name: "cloudwatch_keepStream",
+			b: &batch{
+				streams: map[string]*logproto.Stream{},
+			},
+			expectedStream: `{__aws_cloudwatch_log_group="testLogGroup", __aws_cloudwatch_log_stream="testLogStream", __aws_cloudwatch_owner="123456789123", __aws_log_type="cloudwatch"}`,
+			keepStream:     true,
+		},
+	}
+
+	for _, tt := range tests {
+		// Docs: https://docs.aws.amazon.com/lambda/latest/dg/services-cloudwatchlogs.html
+		// Example CloudWatchLogEvent copied from https://github.com/aws/aws-lambda-go/blob/main/events/cloudwatch_logs_test.go
+		cwevent := &events.CloudwatchLogsEvent{
+			AWSLogs: events.CloudwatchLogsRawData{
+				Data: "H4sIAAAAAAAAAHWPwQqCQBCGX0Xm7EFtK+smZBEUgXoLCdMhFtKV3akI8d0bLYmibvPPN3wz00CJxmQnTO41whwWQRIctmEcB6sQbFC3CjW3XW8kxpOpP+OC22d1Wml1qZkQGtoMsScxaczKN3plG8zlaHIta5KqWsozoTYw3/djzwhpLwivWFGHGpAFe7DL68JlBUk+l7KSN7tCOEJ4M3/qOI49vMHj+zCKdlFqLaU2ZHV2a4Ct/an0/ivdX8oYc1UVX860fQDQiMdxRQEAAA==",
+			},
+		}
+
+		t.Run(tt.name, func(t *testing.T) {
+			batchSize = 131072 // Set large enough we don't send to promtail
+			keepStream = tt.keepStream
+			err := parseCWEvent(context.Background(), tt.b, cwevent)
+			if err != nil {
+				t.Error(err)
+			}
+			require.Len(t, tt.b.streams, 1)
+			stream, ok := tt.b.streams[tt.expectedStream]
+			require.True(t, ok, "batch does not contain stream: %s", tt.expectedStream)
+			require.NotNil(t, stream)
+		})
+	}
+}