fix(receiver/prometheusremotewrite): count target_info samples in PRW response stats (#47108)

<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description
> "we got 2xx status code from the Receiver yet statistics indicate some
data was not written"

This is due to target_info not being counted in samples leading to miss
match count as mentioned in the
[commet](#45150 (comment))
 
<!-- Issue number (e.g. #1234) or full URL to issue, if applicable. -->
#### Link to tracking issue
Fixes #45150 

<!--Describe what testing was performed and which tests were added.-->
#### Testing
Added Sub-Test in TestTranslateV2 with 
- `service with only target_info metric` to verify stats are counted
when target_info
- Verified exising test `service with target_info metric` passes 

<!--Please delete paragraphs that you did not use before submitting.-->

---------

Signed-off-by: Naman-B-Parlecha <naman.parlecha@finalroundai.com>
Signed-off-by: Naman-B-Parlecha <namanparlecha@gmail.com>
Co-authored-by: David Ashpole <dashpole@google.com>
Co-authored-by: Yang Song <songy23@users.noreply.github.com>

Author: 3 people

.chloggen/47108.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. receiver/filelog)
+component: receiver/prometheus_remote_write
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Count target_info samples in PRW response stats
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [47108]
+
+# (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/prometheusremotewritereceiver/receiver.go

@@ -342,6 +342,9 @@ func (prw *prometheusRemoteWriteReceiver) translateV2(_ context.Context, req *wr
 			snapshot := pmetric.NewResourceMetrics()
 			attrs.CopyTo(snapshot.Resource().Attributes())
 			prw.rmCache.Add(hashed, snapshot)
+			// target_info is not stored as a metric but PRW requires the response
+			// to report all received samples, including target_info, to avoid a stats mismatch
+			stats.Samples += len(ts.Samples)
 			continue
 		}
 

receiver/prometheusremotewritereceiver/receiver_test.go

@@ -2255,6 +2255,48 @@ func TestTranslateV2(t *testing.T) {
 				Confirmed: true,
 			},
 		},
+		{
+			name: "service with only target_info metric",
+			request: &writev2.Request{
+				Symbols: []string{
+					"",
+					"job", "production/service_a", // 1, 2
+					"instance", "host1", // 3, 4
+					"machine_type", "n1-standard-1", // 5, 6
+					"cloud_provider", "gcp", // 7, 8
+					"region", "us-central1", // 9, 10
+					"datacenter", "sdc", // 11, 12
+					"__name__", "target_info", // 13, 14
+				},
+				Timeseries: []writev2.TimeSeries{
+					{
+						// target_info metric
+						Metadata:   writev2.Metadata{Type: writev2.Metadata_METRIC_TYPE_GAUGE},
+						LabelsRefs: []uint32{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14},
+						Samples:    []writev2.Sample{{Value: 1, Timestamp: 1}},
+					},
+				},
+			},
+			expectedMetrics: func() pmetric.Metrics {
+				metrics := pmetric.NewMetrics()
+				rm := metrics.ResourceMetrics().AppendEmpty()
+				attrs := rm.Resource().Attributes()
+				attrs.PutStr("service.namespace", "production")
+				attrs.PutStr("service.name", "service_a")
+				attrs.PutStr("service.instance.id", "host1")
+				attrs.PutStr("machine_type", "n1-standard-1")
+				attrs.PutStr("cloud_provider", "gcp")
+				attrs.PutStr("region", "us-central1")
+				attrs.PutStr("datacenter", "sdc")
+				return metrics
+			}(),
+			expectedStats: remote.WriteResponseStats{
+				Confirmed:  true,
+				Samples:    1,
+				Histograms: 0,
+				Exemplars:  0,
+			},
+		},
 	} {
 		t.Run(tc.name, func(t *testing.T) {
 			// since we are using the rmCache to store values across requests, we need to clear it after each test, otherwise it will affect the next test