[statsdreceiver] support simple tags

dogstatsd supports two types of tags on metrics: simple and
dimensional tags[^1]. the former is just a key, the latter is
a key and a value.

with the assumption that many users of the statsdreceiver are
enabling ingest of dogstatsd metrics, this makes the statsd
parser more optimistic, so it can handle tags w/o a value.

when this happens, we set an attribute that has a zero value.
so far as i know, this is allowed in the opentelemetry spec.
the decision of how to handle attributes w/ zero values is
best left to configuration w/in the pipeline itself, as different
users may have different opinions or approaches that work best
with their systems.

[^1]: https://www.datadoghq.com/blog/the-power-of-tagged-metrics/#whats-a-metric-tag

receiver/statsdreceiver/internal/protocol/statsd_parser.go

@@ -410,11 +410,18 @@ func parseMessageToMetric(line string, enableMetricType bool) (statsDMetric, err
 
 			for _, tagSet := range tagSets {
 				tagParts := strings.SplitN(tagSet, ":", 2)
-				if len(tagParts) != 2 {
-					return result, fmt.Errorf("invalid tag format: %s", tagParts)
-				}
 				k := tagParts[0]
-				v := tagParts[1]
+				if k == "" {
+					return result, fmt.Errorf("invalid tag format: %q", tagSet)
+				}
+
+				// support both simple tags (w/o value) and dimension tags (w/ value).
+				// dogstatsd notably allows simple tags.
+				var v string
+				if len(tagParts) == 2 {
+					v = tagParts[1]
+				}
+
 				kvs = append(kvs, attribute.String(k, v))
 			}
 		default:

receiver/statsdreceiver/internal/protocol/statsd_parser_test.go

@@ -53,8 +53,8 @@ func Test_ParseMessageToMetric(t *testing.T) {
 		},
 		{
 			name:  "invalid tag format",
-			input: "test.metric:42|c|#key1",
-			err:   errors.New("invalid tag format: [key1]"),
+			input: "test.metric:42|c|#:val1",
+			err:   errors.New("invalid tag format: \":val1\""),
 		},
 		{
 			name:  "unrecognized message part",
@@ -90,7 +90,7 @@ func Test_ParseMessageToMetric(t *testing.T) {
 			err:   errors.New("unsupported metric type: unhandled_type"),
 		},
 		{
-			name:  "counter metric with sample rate and tag",
+			name:  "counter metric with sample rate and (dimensional) tag",
 			input: "test.metric:42|c|@0.1|#key:value",
 			wantMetric: testStatsDMetric(
 				"test.metric",
@@ -101,6 +101,18 @@ func Test_ParseMessageToMetric(t *testing.T) {
 				[]string{"key"},
 				[]string{"value"}),
 		},
+		{
+			name:  "counter metric with sample rate and (simple) tag",
+			input: "test.metric:42|c|@0.1|#key",
+			wantMetric: testStatsDMetric(
+				"test.metric",
+				42,
+				false,
+				"c",
+				0.1,
+				[]string{"key"},
+				[]string{""}),
+		},
 		{
 			name:  "counter metric with sample rate(not divisible) and tag",
 			input: "test.metric:42|c|@0.8|#key:value",