Add Jaeger Prometheus metrics

[tiagonapoli]

What is the purpose of this pull request?

• Add Jaeger Prometheus metrics (https://github.com/jaegertracing/jaeger-client-node/blob/master/src/metrics/metrics.js)
• Fix typo on request abort event (the event is 'aborted', not 'abort').
• Fix waiting for outgoing response to finish: The current node implementation for stream.finish doesn't detect a destroyed ServerResponse - if the request is aborted, the response stream will be destroyed and then calling stream.finish with a destroyed ServerResponse won't trigger the callback. I opened a issue on NodeJS and hopefully we will get this feature in the next major :D (stream.finished failing to detect a destroyed ServerResponse nodejs/node#34301 stream: finished on closed OutgoingMessage nodejs/node#34313), but for now I created a workaround

What problem is this solving?

We don't have metrics on spans and traces generation per service. This will help us to detect if a service is overflowing Jaeger.

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Requires change to documentation, which has been updated accordingly.