Add optional custom properties to logging messages.

[ivaramme]

The idea behind this PR is to allow users to track additional information to logging actions by simply adding to the default properties, any additional metadata passed by the user in the form of a dictionary.

The usage of this would look like this:

import logging

from opencensus.ext.azure.log_exporter import AzureLogHandler

logger = logging.getLogger(__name__)
# TODO: set APPLICATIONINSIGHTS_CONNECTION_STRING as environment variable.
logger.addHandler(AzureLogHandler())
logger.warning('action', {'key-1': 'value-1', 'key-2': 'value2'})

Once this data is uploaded to App Insights, you can search in the traces table by keys included in the customDimensions column like this:

traces
| where customDimensions.key-1 == "value-1"
| limit 50
| order by timestamp desc 

This is a good option instead of using Custom Events which is not currently supported in Open Census.

[lzchen]

Looks good! Just a few comments.