make Sample namedtuple use Dict instead of Mapping

Signed-off-by: Derek Kulinski <d@kulinski.us>

prometheus_client/metrics.py

@@ -218,8 +218,7 @@ def _multi_samples(self):
             for suffix, sample_labels, value, timestamp, exemplar in metric._samples():
                 yield (suffix, dict(series_labels + list(sample_labels.items())), value, timestamp, exemplar)
 
-    def _child_samples(self):  # pragma: no cover
-        # type: () -> Sequence[Tuple[str, Dict[str, str], float]]
+    def _child_samples(self) -> Sequence[Tuple[str, Dict[str, str], float]]:  # pragma: no cover
         raise NotImplementedError('_child_samples() must be implemented by %r' % self)
 
     def _metric_init(self):  # pragma: no cover

prometheus_client/samples.py

@@ -1,5 +1,4 @@
-from collections import namedtuple
-from typing import Mapping, NamedTuple, Optional, Sequence
+from typing import Dict, NamedTuple, Optional
 
 
 class Timestamp:
@@ -38,14 +37,14 @@ def __gt__(self, other):
 # a Timestamp object, or None.
 # Exemplar can be an Exemplar object, or None.
 class Exemplar(NamedTuple):
-    labels: Mapping[str, str]
+    labels: Dict[str, str]
     value: float
     timestamp: Optional[float] = None
 
 
 class Sample(NamedTuple):
     name: str
-    labels: Mapping[str, str]
+    labels: Dict[str, str]
     value: float
     timestamp: Optional[float] = None
     exemplar: Optional[Exemplar] = None