Fix operator precedence allowing exemplars on any metric type

_is_valid_exemplar_metric() guarded histograms with

    if metric.type in ('histogram') and sample.name.endswith('_bucket') or sample.name == metric.name:

Because 'and' binds tighter than 'or', this parsed as

    (metric.type in ('histogram') and sample.name.endswith('_bucket')) or (sample.name == metric.name)

so the trailing 'sample.name == metric.name' clause fired for every
metric type. As a result the OpenMetrics writer emitted exemplars on
gauges, info, stateset, summary and untyped metrics whenever a sample
name equalled the metric name -- output that the library's own
OpenMetrics parser rejects (only histogram/gaugehistogram buckets,
counter _total, and native histograms may carry exemplars).

Group the histogram condition correctly so the same-name clause (which
exists to allow native-histogram exemplars) only applies to histograms.
While here, replace the 'metric.type in (...)' single-string membership
checks with '==' -- they were doing substring matching, not the
intended equality.

Add test_gauge_exemplar, which asserts a gauge sample carrying an
exemplar raises ValueError, matching the existing untyped/non-bucket
exemplar tests.

Signed-off-by: Sean Kim <skim8705@gmail.com>

Author: sean-kim05

prometheus_client/openmetrics/exposition.py

@@ -25,9 +25,9 @@
 def _is_valid_exemplar_metric(metric, sample):
     if metric.type == 'counter' and sample.name.endswith('_total'):
         return True
-    if metric.type in ('gaugehistogram') and sample.name.endswith('_bucket'):
+    if metric.type == 'gaugehistogram' and sample.name.endswith('_bucket'):
         return True
-    if metric.type in ('histogram') and sample.name.endswith('_bucket') or sample.name == metric.name:
+    if metric.type == 'histogram' and (sample.name.endswith('_bucket') or sample.name == metric.name):
         return True
     return False
 

tests/openmetrics/test_exposition.py

@@ -347,6 +347,21 @@ def collect(self):
         with self.assertRaises(ValueError):
             generate_latest(self.registry)
 
+    def test_gauge_exemplar(self) -> None:
+        class MyCollector:
+            def collect(self):
+                metric = Metric("gg", "A gauge", 'gauge')
+                # A sample whose name equals the metric name must not be
+                # treated as exemplar-eligible just because it matches;
+                # only histogram/gaugehistogram buckets, counter _total, and
+                # native histograms may carry exemplars.
+                metric.add_sample("gg", {}, 1, None, Exemplar({'a': 'b'}, 0.5))
+                yield metric
+
+        self.registry.register(MyCollector())
+        with self.assertRaises(ValueError):
+            generate_latest(self.registry)
+
     def test_gaugehistogram(self) -> None:
         self.custom_collector(
             GaugeHistogramMetricFamily('gh', 'help', buckets=[('1.0', 4), ('+Inf', (5))], gsum_value=7))