ci: Add __name__ to OTel naming check Rule D builtins

Rule D (dashboard PromQL labels must exist in L1) flagged `__name__` once the
phase-7 system-*.json dashboards started using `sum by (le, __name__)`.
`__name__` is the Prometheus reserved label for the metric name itself — a
builtin, not a span attribute. Add it to the builtin allowlist and cover it
with a test. (Earlier dashboards only used `__name__` inside `{__name__=~...}`
matchers, which the label regex did not extract, so this surfaced only now.)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.github/scripts/otel-naming/check_otel_naming.py

@@ -699,6 +699,7 @@ def run_rule_d_dashboards(root: Path, l1_keys: Set[str], report: Report) -> None
         report.skip("D", "no L1 key set to validate against")
         return
     builtins = {
+        "__name__",  # Prometheus reserved label for the metric name itself
         "le",
         "exported_instance",
         "span_name",

.github/scripts/otel-naming/test_check_otel_naming.py

@@ -502,6 +502,14 @@ class RuleDDashboards(unittest.TestCase):
             [],
         )
 
+    def test_prometheus_name_label_not_flagged(self):
+        # `__name__` is the Prometheus reserved metric-name label; the renamed
+        # system-*.json dashboards use `sum by (le, __name__)`.
+        self.assertEqual(
+            self._run('"expr": "sum by (le, __name__) (rate(x[5m]))"', set()),
+            [],
+        )
+
     def test_l1_label_passes(self):
         self.assertEqual(self._run('"q": "{command=\\"x\\"}"', {"command"}), [])