tests: create a test suite which will configure prometheus for etcd/fleet metrics monitoring

[kayrus]

Then we can combine this test with functional tests and as a result have a rough
performance measurements.

/cc @jonboulle as usual your comments are welcome.

relates to: #1415

[jonboulle]

Sure but let's get the metrics landed first :-)

[dongsupark]

Just FYI, prometheus metrics were already added via #1524