test(runtime-metrics): test env variable (#2567)

Add a test case to ensure that ddtrace-run behaves as expected based on
the value of the DD_RUNTIME_METRICS_ENABLED environment variable.

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

Author: P403n1x87

tests/runtime/test_runtime_metrics_api.py

@@ -27,7 +27,7 @@ def test_runtime_metrics_api_idempotency():
 def test_manually_start_runtime_metrics(run_python_code_in_subprocess):
     """
     When importing and manually starting runtime metrics
-        There are no errors
+        Runtime metrics worker starts and there are no errors
     """
     out, err, status, pid = run_python_code_in_subprocess(
         """
@@ -43,3 +43,27 @@ def test_manually_start_runtime_metrics(run_python_code_in_subprocess):
     )
 
     assert status == 0
+
+
+def test_start_runtime_metrics_via_env_var(ddtrace_run_python_code_in_subprocess):
+    """
+    When running with ddtrace-run and DD_RUNTIME_METRICS_ENABLED is set
+        Runtime metrics worker starts and there are no errors
+    """
+
+    _, _, status, _ = ddtrace_run_python_code_in_subprocess(
+        """
+from ddtrace.internal.runtime.runtime_metrics import RuntimeWorker
+assert RuntimeWorker._instance is None
+"""
+    )
+    assert status == 0
+
+    _, _, status, _ = ddtrace_run_python_code_in_subprocess(
+        """
+from ddtrace.internal.runtime.runtime_metrics import RuntimeWorker
+assert RuntimeWorker._instance is not None
+""",
+        env={"DD_RUNTIME_METRICS_ENABLED": "true"},
+    )
+    assert status == 0