fix: datadog count metrics (influxdata#10979)

(cherry picked from commit eb79136)

plugins/outputs/datadog/README.md

@@ -32,5 +32,11 @@ key with a `.` character.
 Field values are converted to floating point numbers.  Strings and floats that
 cannot be sent over JSON, namely NaN and Inf, are ignored.
 
+We do not send `Rate` types. Counts are sent as `count`, with an
+interval hard-coded to 1. Note that this behavior does *not* play
+super-well if running simultaneously with current Datadog agents; they
+will attempt to change to `Rate` with `interval=10`. We prefer this
+method, however, as it reflects the raw data more accurately.
+
 [metrics]: https://docs.datadoghq.com/api/v1/metrics/#submit-metrics
 [apikey]: https://app.datadoghq.com/account/settings#api

plugins/outputs/datadog/datadog.go

@@ -51,10 +51,12 @@ type TimeSeries struct {
 }
 
 type Metric struct {
-	Metric string   `json:"metric"`
-	Points [1]Point `json:"points"`
-	Host   string   `json:"host"`
-	Tags   []string `json:"tags,omitempty"`
+	Metric   string   `json:"metric"`
+	Points   [1]Point `json:"points"`
+	Host     string   `json:"host"`
+	Type     string   `json:"type,omitempty"`
+	Tags     []string `json:"tags,omitempty"`
+	Interval int64    `json:"interval"`
 }
 
 type Point [2]float64
@@ -103,10 +105,21 @@ func (d *Datadog) Write(metrics []telegraf.Metric) error {
 				} else {
 					dname = m.Name() + "." + fieldName
 				}
+				var tname string
+				switch m.Type() {
+				case telegraf.Counter:
+					tname = "count"
+				case telegraf.Gauge:
+					tname = "gauge"
+				default:
+					tname = ""
+				}
 				metric := &Metric{
-					Metric: dname,
-					Tags:   metricTags,
-					Host:   host,
+					Metric:   dname,
+					Tags:     metricTags,
+					Host:     host,
+					Type:     tname,
+					Interval: 1,
 				}
 				metric.Points[0] = dogM
 				tempSeries = append(tempSeries, metric)