Expose prometheus metrics

[kadaan]

what

• Exposes metrics from the prometheus-to-cloudwatch service via the standard prometheus scrape endpoint.
• Creates the following metrics:
  • Total Publishes
  • Total Publish Errors
  • Publish Duration
  • Total Metrics
• New port chosen that doesn't collide with existing prometheus exporters.

why

• There is no current ability to monitor the prometheus-to-cloudwatch service
• This exposes total and error metrics that can allow prometheus alerts to be created that will notify based on failure rate or hung process.
• Monitoring is needed to be able to rely on this service in production environments

references

• Verify port choice on https://github.com/prometheus/prometheus/wiki/Default-port-allocations and update this with port when merged.

[aknysh]

@kadaan thanks for the PR.
Looks like you are adding a server and prometheus to be able to get metrics from this app itself, correct?
Can you please add more description to the PR in the form:

## what
* Describe high-level what changed as a result of these commits (i.e. what do these changes mean)
* Use bullet points to be concise and to the point

## why
* Provide the justifications for the changes (e.g. business case)
* Describe why these changes were made (e.g. why do these commits fix the problem)
* Use bullet points to be concise and to the point

## references
* Link to any supporting GitHub issues or helpful documentation to add some context (e.g. stackoverflow)
* Use `closes #123`, if this PR closes a GitHub issue `#123`

[kadaan]

@aknysh Updated