Allow passing a namespace with a custom metric

[yoranvanoirschot]

Is your feature request related to a problem? Please describe.
Currently all custom metrics land in the namespace: 'azure.applicationinsights'. It seems to be possible to provide a Metric namespace through the REST API (https://docs.microsoft.com/en-us/azure/azure-monitor/essentials/metrics-custom-overview#sample-custom-metric-publication) and in the C# SDK (https://github.com/microsoft/ApplicationInsights-dotnet/blob/405fd6a9916956f2233520c8ab66110a1f9dcfbc/BASE/src/Microsoft.ApplicationInsights/DataContracts/MetricTelemetry.cs) however this is unavailable from the Java SDK.

Describe the solution you would like
I would like to set the namespace in a similar way to the C# SDK, an example:

var metric = new MetricTelemetry();
metric.setMetricNamespace("<myNamespace>");
metric.setName("<myMetric>");
metric.setValue(50.0);
client.trackMetric(metric);

Preferably we would be able to set a default metric namespace on the TelemetryClient.

Describe alternatives you have considered
Alternatively we prefix the metric name with the namespace. We are using a shared App Insights instance for over 40 applications. This will result in a huge list of metrics. We would rather namespace them per application (or per group of applications).