diff --git a/agent-inject/metrics_test.go b/agent-inject/metrics_test.go
new file mode 100644
index 00000000..65101877
--- /dev/null
+++ b/agent-inject/metrics_test.go
@@ -0,0 +1,67 @@
+// Copyright (c) HashiCorp, Inc.
+// SPDX-License-Identifier: MPL-2.0
+
+package agent_inject
+
+import (
+	"testing"
+
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/testutil"
+	"github.com/stretchr/testify/assert"
+)
+
+func Test_incrementInjections(t *testing.T) {
+	MustRegisterInjectorMetrics(prometheus.DefaultRegisterer)
+
+	tests := map[string]struct {
+		namespace      string
+		mutateResponse MutateResponse
+		expectedLabels map[string]string
+	}{
+		"init_only": {
+			namespace: "init",
+			mutateResponse: MutateResponse{
+				InjectedInit:    true,
+				InjectedSidecar: false,
+			},
+			expectedLabels: map[string]string{
+				metricsLabelNamespace: "init",
+				metricsLabelType:      metricsLabelTypeInitOnly,
+			},
+		},
+		"sidecar_only": {
+			namespace: "sidecar",
+			mutateResponse: MutateResponse{
+				InjectedInit:    false,
+				InjectedSidecar: true,
+			},
+			expectedLabels: map[string]string{
+				metricsLabelNamespace: "sidecar",
+				metricsLabelType:      metricsLabelTypeSidecarOnly,
+			},
+		},
+		"init_and_sidecar": {
+			namespace: "both",
+			mutateResponse: MutateResponse{
+				InjectedInit:    true,
+				InjectedSidecar: true,
+			},
+			expectedLabels: map[string]string{
+				metricsLabelNamespace: "both",
+				metricsLabelType:      metricsLabelTypeBoth,
+			},
+		},
+	}
+	for name, test := range tests {
+		t.Run(name, func(t *testing.T) {
+			t.Cleanup(func() {
+				injectionsByNamespace.Reset()
+			})
+			incrementInjections(test.namespace, test.mutateResponse)
+			assert.Equal(t, 1, testutil.CollectAndCount(injectionsByNamespace))
+			check := injectionsByNamespace.With(prometheus.Labels(test.expectedLabels))
+			assert.Equal(t, float64(1), testutil.ToFloat64(check))
+		})
+	}
+}