[receiver/statsdreceiver] Gracefully handle empty tagset (open-teleme…

…try#27011)

We have a set of client libraries
(`com.timgroup.statsd.NonBlockingStatsDClient`) that send a metric like
this where there are no tags:

```
some_metric_name:1|c|#
```

Now the validity of this StatsD message can be questioned but the fact
of the matter is this is widespread/common behavior for now. This PR
includes a fix to move on if no tags in the array are present.

.chloggen/cj-handle-empty-tagset.yaml

@@ -0,0 +1,27 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: enhancement
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: statsdreceiver
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Allow for empty tag sets
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [27011]
+
+# (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:
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: []

receiver/statsdreceiver/internal/protocol/statsd_parser.go

@@ -400,6 +400,12 @@ func parseMessageToMetric(line string, enableMetricType bool) (statsDMetric, err
 		case strings.HasPrefix(part, "#"):
 			tagsStr := strings.TrimPrefix(part, "#")
 
+			// handle an empty tag set
+			// where the tags part was still sent (some clients do this)
+			if len(tagsStr) == 0 {
+				continue
+			}
+
 			tagSets := strings.Split(tagsStr, ",")
 
 			for _, tagSet := range tagSets {

receiver/statsdreceiver/internal/protocol/statsd_parser_test.go

@@ -61,6 +61,15 @@ func Test_ParseMessageToMetric(t *testing.T) {
 			input: "test.metric:42|c|$extra",
 			err:   errors.New("unrecognized message part: $extra"),
 		},
+		{
+			name:  "integer counter with no tags",
+			input: "test.metric:42|c|#",
+			wantMetric: testStatsDMetric(
+				"test.metric",
+				42,
+				false,
+				"c", 0, nil, nil),
+		},
 		{
 			name:  "integer counter",
 			input: "test.metric:42|c",