fix(metrics): explicit type to single_metric ctx manager (aws-powertools#865)

Co-authored-by: Heitor Lessa <[email protected]>

Author: whardier

aws_lambda_powertools/metrics/metric.py

@@ -1,7 +1,7 @@
 import json
 import logging
 from contextlib import contextmanager
-from typing import Dict, Optional, Union
+from typing import Dict, Optional, Union, Generator
 
 from .base import MetricManager, MetricUnit
 
@@ -61,7 +61,7 @@ def add_metric(self, name: str, unit: Union[MetricUnit, str], value: float) -> N
 
 
 @contextmanager
-def single_metric(name: str, unit: MetricUnit, value: float, namespace: Optional[str] = None):
+def single_metric(name: str, unit: MetricUnit, value: float, namespace: Optional[str] = None) -> Generator[SingleMetric, None, None]:
     """Context manager to simplify creation of a single metric
 
     Example