Figure out Namespacing Support for Metrics

[felixnext]

The MetricHandler in current state is a Singleton object. Meaning it will use one MeasurementMap for all metrics.
In most cases (esp in Azure Functions) this should not be a problem, since we only operate with one set of metrics at a time.

There might be edge cases though, where we want to have support for multiple MeasurementMaps? (Confirm that). If that is the case it would make sense to change the self._map module from a direct instance to a Dict[str, MeasurementMap] and create a default namespace.

If create_metrics is called with no namespace parameter this will use the default namespace, otherwise check if the namespace exists and if not create a new MeasurementMap.

This namespace parameter would then also need to be exposed to the Decorator. It might also require to create a thin wrapper object around MetricHandler (e.g. MetricNamespaceHandler or MetricNSHandler) to auto-pass the namespace for interactions with metrics inside the annotated function.

Bonus: We might also want to add a __namespace column automatically to all metrics and pass this information along in the TagMap.

Help Wanted

Please specify if this is a functionality that is really required!

[felixnext]

Closing this with #10 - After consideration how the MeasurementMap works namespaces are integrated into the MetricSpec