Azure: Allow specifying metrics for custom events

[Davi-DeGanne]

Description

This PR adds the ability to specify custom measurements as part of Azure events, emulating the way it was done for custom dimensions.

Testing

I tested this manually using the following python command:

import logging
from opencensus.ext.azure.log_exporter import AzureEventHandler

events = logging.getLogger("myLogger")
events.addHandler(AzureEventHandler())
events.warning("My event", extra={"custom_dimensions":{"type":"Example"},"custom_measurements":{"example_metric":1}})

My Application Insights resource successfully received a custom event with both the customDimensions and customMeasurements properties.

Additional Context

This was requested in a comment in #809 and this PR might be the final piece needed to close that issue, but I'm not sure because the feature request made there isn't very specific.

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[lzchen]

Please add a CHANGELOG entry.

[lzchen]

If you rebase, I believe the build errors will be fixed.

[Davi-DeGanne]

@lzchen Ok, how does this look?