Support Azure Monitor custom metric namespace

[louisfelix]

Hi @brian-brazil !

Azure Monitor API have a metric namespace concept that is not supported in the exporter. In Azure Monitor API, metrics are returned for the default namespace, but a custom namespace can be used for custom metrics. One of the use case we need this feature for is to access the guest OS performance counters, but the goal here is to support custom metrics in general (a custom namespace must be passed to the API query in order to access those).

This PR adds the following:

• a new optional metric_namespace property to specify namespace of the metrics to gather
• a new --list.namespaces CLI flag to list available metric namespaces for the given resources
• modified --list.definitions to support the optional namespace when listing available metric definitions for the given resources

Thanks!

[louisfelix]

Shouldn't this be automatically included in the output metric name?

Yes, the metric namespace could be added as a prefix of the metric name (like in the cloudwatch exporter) if it is specified. Is that what you mean?

[brian-brazil]

Is that what you mean?

Yes

[louisfelix]

Hi @brian-brazil , I fixed all review items.

[brian-brazil]

Thanks!