Add support for Flink cluster metrics to PerfKit Benchmarker

[kennknowles]

See https://docs.google.com/document/d/1PsjGPSN6FuorEEPrKEP3u3m16tyOzph5FnL2DhaRDz0/edit?ts=58a78e73#heading=h.exn0s6jsm24q for more details on what this entails.

Blocked on BEAM-1601, adding support for Flink to PKB.

Imported from Jira BEAM-1607. Original Jira may contain additional context.
Reported by: jaku.