Drop metrics with mixed tagging styles

Signed-off-by: Tony Wooster <twooster@gmail.com>

README.md

@@ -65,16 +65,12 @@ metric.name:0|c|#tagName=val,tag2Name=val2
 See [Tags](https://docs.datadoghq.com/developers/dogstatsd/data_types/#tagging)
 in the DogStatsD documentation for the concept description and
 [Datagram Format](https://docs.datadoghq.com/developers/dogstatsd/datagram_shell/).
-Note that this tagging style is incompatible with the original `statsd`
-implementation, you will be unable to use it as a repeater in front of the
-statsd-to-prometheus exporter.
+If you encounter problems, note that this tagging style is incompatible with
+the original `statsd` implementation.
 
-Although you can use both name-appended tags (Librato or InfluxDB) and
-metric-appended tags simultaneously, this is not recommended. If you do, be
-aware that DogStatsD `name=value` pairs will take priority over Librato tags
-with the same name.
-
-Tags without values (`#some_tag`) are not supported and will be dropped.
+Be aware: If you mix tag styles (e.g., Librato/InfluxDB with DogStatsD), the
+exporter will consider this an error and the sample will be discarded. Also,
+tags without values (`#some_tag`) are not supported and will be ignored.
 
 ## Building and Running
 

bridge_test.go

@@ -238,15 +238,17 @@ func TestHandlePacket(t *testing.T) {
 				},
 			},
 		}, {
-			name: "librato and datadog tag extension with sampling",
+			name: "librato/dogstatsd mixed tag styles without sampling",
+			in:   "foo#tag1=foo,tag3=bing:100|c|#tag1:bar,#tag2:baz",
+			out:  Events{},
+		}, {
+			name: "influxdb/dogstatsd mixed tag styles without sampling",
+			in:   "foo,tag1=foo,tag3=bing:100|c|#tag1:bar,#tag2:baz",
+			out:  Events{},
+		}, {
+			name: "mixed tag styles with sampling",
 			in:   "foo#tag1=foo,tag3=bing:100|c|@0.1|#tag1:bar,#tag2:baz",
-			out: Events{
-				&CounterEvent{
-					metricName: "foo",
-					value:      1000,
-					labels:     map[string]string{"tag1": "bar", "tag3": "bing", "tag2": "baz"},
-				},
-			},
+			out:  Events{},
 		}, {
 			name: "histogram with sampling",
 			in:   "foo:0.01|h|@0.2|#tag1:bar,#tag2:baz",

exporter.go

@@ -399,9 +399,19 @@ func lineToEvents(line string) Events {
 
 	labels := map[string]string{}
 	metric := parseNameAndLabels(elements[0], labels)
+
 	var samples []string
 	if strings.Contains(elements[1], "|#") {
-		// using datadog extensions, disable multi-metrics
+		// using datadog extensions
+
+		// don't allow mixed tagging styles
+		if len(labels) > 0 {
+			sampleErrors.WithLabelValues("malformed_line").Inc()
+			log.Debugln("Bad line (multiple tagging styles) from StatsD:", line)
+			return events
+		}
+
+		// disable multi-metrics
 		samples = elements[1:]
 	} else {
 		samples = strings.Split(elements[1], ":")