Add support for metrics with Promotheus about rabbitmq-perf-test

[acogoluegnes]

Monitoring backend configuration would follow the pattern --metrics.<backend>.configuration=<key1>=<value1>,<key2>=<value2>, e.g. --metrics.datadog.configuration=apiKey=a34349a3434a,applicationKey=e343ea343

For each backend, there could be specific cases, e.g. the hostname tag for Datadog, that can be fed by default with the value the JVM returns (it could also be set up on the command line).

Tags could be set up e.g. --metrics-tags=dc=us,environment=production.

Several backends can be combined by specifying their respecting command line options and using CompositeMeterRegistry internally.

Regarding dependencies, it may be overkill to embed the dependencies of all backend systems: some of them are lightweight (e.g. for Datadog, only the Micrometer Datadog module) and others can drag several transitives dependencies (e.g. Atlas). If embedding the dependencies double or triple the size of the current PerfTest, we may provide some scripts to download the backend dependencies the user wants.

[acogoluegnes]

This issue will be closed by #99, which brings the metrics infrastructure (API, command lines, etc) and support for Prometheus. Other issues will be created for other backends support.