Support setting process-wide tags in JaegerExporter constructor

[TBBle]

Is your feature request related to a problem?

In order to work-around jaegertracing/jaeger#1459, we need to add an 'ip' process tag, as Jaeger's Clock Skew Adjustor assumes untagged processes come from different machines, and messes with the timestamps if child spans don't form proper nested subsets.

There's probably other 'process' tags that would be useful, but this is what triggered the need for me.

Describe the solution you'd like.

When instantiating the JaegerExporter, where we pass in the Service Name, I'd like to pass in a python map of process-wide tags as well.

This is consistent with Jaeger's own Python Client, and the OpenCensus Jaeger Exporter's API.

Describe alternatives you've considered.

Since we create the relevant Jaeger object (Process) each time we emit a batch, the process-wide tags could be set after construction of the JaegerExporter.

Additional context.

As a work-around I'm currently monkey-patching the generated Process class to add the ip tag by default, hard-coded to a constant value so that the Clock Skew Adjustor in Jaeger does not corrupt my traces.

Longer-term, it might be work including a few process-wide tags by default (client library version, host IP, hostname), as can be seen in the Jaeger Python client (see references to self.tags).