[processor/transform] use default unit in extract_count_metric() (open-telemetry#31636)

**Description:** 

The `transformprocessor` offers a function `extract_count_metric()`.
Currently this function copies over the `unit` from the original metric
to a new `count` metric. However, this unit is not applicable as the
value is a count of how many values were recorded in a Explicit
Bucket/Exponential Histogram or Summary.

Therefore this PR changes that function so that it adds the default unit
(`1`) to the extracted count metric instead.

**Link to tracking Issue:** open-telemetry#31575 

**Testing:** Unit tests (updated)

**Documentation:** Added a changelog entry as the change is user-facing
(the unit of the emitted telemetry is changed)

---------

Co-authored-by: Tyler Helmuth <[email protected]>

Author: 2 people

.chloggen/fix_extract-count-default-unit.yaml

@@ -0,0 +1,29 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: "bug_fix"
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: transformprocessor
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Change metric unit for metrics extracted with `extract_count_metric()` to be the default unit (`1`)
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [31575]
+
+# (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: |
+  The original metric `unit` does not apply to extracted `count` metrics the same way it does to `sum`, `min` or `max`. 
+  Metrics extracted using `extract_count_metric()` now use the more appropriate default unit (`1`) instead.
+
+# 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: [user]

processor/transformprocessor/internal/metrics/func_extract_count_metric.go

@@ -44,7 +44,8 @@ func extractCountMetric(monotonic bool) (ottl.ExprFunc[ottlmetric.TransformConte
 		countMetric := pmetric.NewMetric()
 		countMetric.SetDescription(metric.Description())
 		countMetric.SetName(metric.Name() + "_count")
-		countMetric.SetUnit(metric.Unit())
+		// Use the default unit as the original metric unit does not apply to the 'count' field
+		countMetric.SetUnit("1")
 		countMetric.SetEmptySum().SetAggregationTemporality(aggTemp)
 		countMetric.Sum().SetIsMonotonic(monotonic)
 

processor/transformprocessor/internal/metrics/func_extract_count_metric_test.go

@@ -24,6 +24,7 @@ func Test_extractCountMetric(t *testing.T) {
 				histogramMetric := getTestHistogramMetric()
 				histogramMetric.CopyTo(metrics.AppendEmpty())
 				countMetric := metrics.AppendEmpty()
+				countMetric.SetUnit("1")
 				countMetric.SetEmptySum()
 				countMetric.Sum().SetAggregationTemporality(histogramMetric.Histogram().AggregationTemporality())
 				countMetric.Sum().SetIsMonotonic(false)
@@ -44,6 +45,7 @@ func Test_extractCountMetric(t *testing.T) {
 				histogramMetric := getTestHistogramMetric()
 				histogramMetric.CopyTo(metrics.AppendEmpty())
 				countMetric := metrics.AppendEmpty()
+				countMetric.SetUnit("1")
 				countMetric.SetEmptySum()
 				countMetric.Sum().SetAggregationTemporality(histogramMetric.Histogram().AggregationTemporality())
 				countMetric.Sum().SetIsMonotonic(true)
@@ -64,6 +66,7 @@ func Test_extractCountMetric(t *testing.T) {
 				expHistogramMetric := getTestExponentialHistogramMetric()
 				expHistogramMetric.CopyTo(metrics.AppendEmpty())
 				countMetric := metrics.AppendEmpty()
+				countMetric.SetUnit("1")
 				countMetric.SetEmptySum()
 				countMetric.Sum().SetAggregationTemporality(expHistogramMetric.ExponentialHistogram().AggregationTemporality())
 				countMetric.Sum().SetIsMonotonic(false)
@@ -85,6 +88,7 @@ func Test_extractCountMetric(t *testing.T) {
 				expHistogramMetric.CopyTo(metrics.AppendEmpty())
 				countMetric := metrics.AppendEmpty()
 				countMetric.SetEmptySum()
+				countMetric.SetUnit("1")
 				countMetric.Sum().SetAggregationTemporality(expHistogramMetric.ExponentialHistogram().AggregationTemporality())
 				countMetric.Sum().SetIsMonotonic(true)
 
@@ -105,6 +109,7 @@ func Test_extractCountMetric(t *testing.T) {
 				summaryMetric.CopyTo(metrics.AppendEmpty())
 				countMetric := metrics.AppendEmpty()
 				countMetric.SetEmptySum()
+				countMetric.SetUnit("1")
 				countMetric.Sum().SetAggregationTemporality(pmetric.AggregationTemporalityCumulative)
 				countMetric.Sum().SetIsMonotonic(false)
 
@@ -125,6 +130,7 @@ func Test_extractCountMetric(t *testing.T) {
 				summaryMetric.CopyTo(metrics.AppendEmpty())
 				countMetric := metrics.AppendEmpty()
 				countMetric.SetEmptySum()
+				countMetric.SetUnit("1")
 				countMetric.Sum().SetAggregationTemporality(pmetric.AggregationTemporalityCumulative)
 				countMetric.Sum().SetIsMonotonic(true)
 

processor/transformprocessor/internal/metrics/processor_test.go

@@ -163,7 +163,7 @@ func Test_ProcessMetrics_MetricContext(t *testing.T) {
 				countMetric.SetName(histogramMetric.Name() + "_count")
 				countMetric.Sum().SetAggregationTemporality(pmetric.AggregationTemporalityDelta)
 				countMetric.Sum().SetIsMonotonic(true)
-				countMetric.SetUnit(histogramMetric.Unit())
+				countMetric.SetUnit("1")
 
 				histogramDp0 := histogramMetric.Histogram().DataPoints().At(0)
 				countDp0 := countMetric.Sum().DataPoints().AppendEmpty()