[procesor/groupbyattrsprocessor]: fix droping metadata when processing metrics (#33781)

**Description:**
Fixes the metadata dropping when processing metrics

**Link to tracking Issue:** #33419 

**Testing:** <Describe what testing was performed and which tests were
added.>
- unit tests

---------

Signed-off-by: odubajDT <[email protected]>

Author: odubajDT

.chloggen/fix-metrics-metadata.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: bug_fix
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: processor/groupbyattrsprocessor
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: "Fix dropping of metadata fields when processing metrics."
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [33419]
+
+# (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: []

processor/groupbyattrsprocessor/processor.go

@@ -206,6 +206,7 @@ func getMetricInInstrumentationLibrary(ilm pmetric.ScopeMetrics, searchedMetric
 	metric.SetDescription(searchedMetric.Description())
 	metric.SetName(searchedMetric.Name())
 	metric.SetUnit(searchedMetric.Unit())
+	searchedMetric.Metadata().CopyTo(metric.Metadata())
 
 	// Move other special type specific values
 	//exhaustive:enforce

processor/groupbyattrsprocessor/processor_test.go

@@ -869,6 +869,59 @@ func TestCompacting(t *testing.T) {
 	}
 }
 
+func Test_GetMetricInInstrumentationLibrary(t *testing.T) {
+	// input metric with datapoint
+	m := pmetric.NewMetric()
+	m.SetName("metric")
+	m.SetDescription("description")
+	m.SetUnit("unit")
+	d := m.SetEmptyGauge().DataPoints().AppendEmpty()
+	d.SetDoubleValue(1.0)
+
+	// expected metric without datapoint
+	// the datapoints are not copied to the resulting metric, since
+	// datapoints are moved in between metrics in the processor
+	m2 := pmetric.NewMetric()
+	m2.SetName("metric")
+	m2.SetDescription("description")
+	m2.SetUnit("unit")
+	m2.SetEmptyGauge()
+
+	metadata := pcommon.NewMap()
+	metadata.PutStr("key", "val")
+	metadata.CopyTo(m.Metadata())
+	metadata.CopyTo(m2.Metadata())
+
+	sm := pmetric.NewScopeMetrics()
+	m.CopyTo(sm.Metrics().AppendEmpty())
+
+	tests := []struct {
+		name     string
+		ilm      pmetric.ScopeMetrics
+		searched pmetric.Metric
+		want     pmetric.Metric
+	}{
+		{
+			name:     "existing metric",
+			ilm:      sm,
+			searched: m,
+			want:     m,
+		},
+		{
+			name:     "non-existing metric - datapoints will be removed",
+			ilm:      pmetric.NewScopeMetrics(),
+			searched: m,
+			want:     m2,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			require.Equal(t, getMetricInInstrumentationLibrary(tt.ilm, tt.searched), tt.want)
+		})
+	}
+}
+
 func BenchmarkCompacting(bb *testing.B) {
 	runs := []struct {
 		ilCount   int