[processor/tailsampling] Fixed sampling policy evaluation debug logging batch metrics (#37040)

#### Description

Currently, the processor always logs (at debug) `sampled=0` and
`notSampled=0` for every batch processed. This pull-request fixes those
metrics.

---------

Signed-off-by: Sean Porter <[email protected]>

Author: portertech

.chloggen/tailsamplingprocessor-fixed-batch-debug-metrics.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: tailsamplingprocessor
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Fixed sampling policy evaluation debug logging batch metrics (e.g. sampled).
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [37040]
+
+# (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: [user]

processor/tailsamplingprocessor/processor.go

@@ -404,6 +404,13 @@ func (tsp *tailSamplingSpanProcessor) makeDecision(id pcommon.TraceID, trace *sa
 		finalDecision = sampling.Sampled
 	}
 
+	switch finalDecision {
+	case sampling.Sampled:
+		metrics.decisionSampled++
+	case sampling.NotSampled:
+		metrics.decisionNotSampled++
+	}
+
 	return finalDecision
 }
 

processor/tailsamplingprocessor/processor_test.go

@@ -609,6 +609,46 @@ func TestDuplicatePolicyName(t *testing.T) {
 	assert.Equal(t, err, errors.New(`duplicate policy name "always_sample"`))
 }
 
+func TestDecisionPolicyMetrics(t *testing.T) {
+	traceIDs, batches := generateIDsAndBatches(10)
+	policy := []PolicyCfg{
+		{
+			sharedPolicyCfg: sharedPolicyCfg{
+				Name:             "test-policy",
+				Type:             Probabilistic,
+				ProbabilisticCfg: ProbabilisticCfg{SamplingPercentage: 50},
+			},
+		},
+	}
+	cfg := Config{
+		DecisionWait:            defaultTestDecisionWait,
+		NumTraces:               uint64(2 * len(traceIDs)),
+		ExpectedNewTracesPerSec: 64,
+		PolicyCfgs:              policy,
+	}
+	sp, _ := newTracesProcessor(context.Background(), processortest.NewNopSettings(), consumertest.NewNop(), cfg)
+	tsp := sp.(*tailSamplingSpanProcessor)
+	require.NoError(t, tsp.Start(context.Background(), componenttest.NewNopHost()))
+	defer func() {
+		require.NoError(t, tsp.Shutdown(context.Background()))
+	}()
+	metrics := &policyMetrics{}
+
+	for i, id := range traceIDs {
+		sb := &sampling.TraceData{
+			ArrivalTime:     time.Now(),
+			ReceivedBatches: batches[i],
+		}
+
+		_ = tsp.makeDecision(id, sb, metrics)
+	}
+
+	assert.EqualValues(t, 5, metrics.decisionSampled)
+	assert.EqualValues(t, 5, metrics.decisionNotSampled)
+	assert.EqualValues(t, 0, metrics.idNotFoundOnMapCount)
+	assert.EqualValues(t, 0, metrics.evaluateErrorCount)
+}
+
 func collectSpanIDs(trace ptrace.Traces) []pcommon.SpanID {
 	var spanIDs []pcommon.SpanID