Report never-set gauges in mostrecent multiprocess mode

In the mostrecent/livemostrecent accumulation branch, sample_timestamps
is a defaultdict(float), so the stored timestamp of a series defaults to
0.0. A gauge child that was created but never set() also has a stored
timestamp of 0, so the strict 'current_timestamp < timestamp' check was
never satisfied and the series was dropped from collection entirely.

Every other gauge mode (sum, min, max, all) reports such a child as 0,
so mostrecent silently omitting it was inconsistent. Register the sample
when it is first seen, regardless of timestamp, and still let a strictly
newer timestamp win afterwards.

Add test_gauge_mostrecent_never_set, asserting a mostrecent gauge that is
created but never set is reported as 0 rather than missing.

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

Author: sean-kim05

prometheus_client/multiprocess.py

@@ -119,7 +119,7 @@ def _accumulate_metrics(metrics, accumulate):
                     elif metric._multiprocess_mode in ('mostrecent', 'livemostrecent'):
                         current_timestamp = sample_timestamps[labels][name]
                         timestamp = float(timestamp or 0)
-                        if current_timestamp < timestamp:
+                        if (name, labels) not in samples[labels] or current_timestamp < timestamp:
                             samples[labels][(name, labels)] = value
                             sample_timestamps[labels][name] = timestamp
                     else:  # all/liveall

tests/test_multiprocess.py

@@ -205,6 +205,16 @@ def test_gauge_livemostrecent(self):
         mark_process_dead(123, os.environ['PROMETHEUS_MULTIPROC_DIR'])
         self.assertEqual(2, self.registry.get_sample_value('g'))
 
+    def test_gauge_mostrecent_never_set(self):
+        # A child created but never set() has a stored timestamp of 0. It must
+        # still be reported as 0, like every other gauge mode, rather than
+        # dropped from collection because 0 is not strictly greater than the
+        # default timestamp of 0.
+        Gauge('g', 'help', registry=None, multiprocess_mode='mostrecent')
+        values.ValueClass = MultiProcessValue(lambda: 456)
+        Gauge('g', 'help', registry=None, multiprocess_mode='mostrecent')
+        self.assertEqual(0, self.registry.get_sample_value('g'))
+
     def test_namespace_subsystem(self):
         c1 = Counter('c', 'help', registry=None, namespace='ns', subsystem='ss')
         c1.inc(1)