Partially addresses #56869:

• Fix etcd-version-monitor to support etcd 3.1: Add support for the etcd 3.1+ go-grpc-prometheus metrics format, which backward incompatibly replaces the 3.0 legacy grpc metric format. Expose the go-grpc-prometheus metrics both in the 3.1 format and in the 3.0 format so systems consuming etcd-version-monitor metrics have a clean, simple upgrade path.
• Expose all etcd metrics by default, making this a one stop shop for all etcd metrics.
• Expose grpc request latency histogram metrics (grpc_server_handling_seconds from go-grpc-prometheus metrics format). Rewrite etcd 3.0 legacy metric for latency histograms to the etcd 3.1+ go-grpc-prometheus format so there is a single format exported for all etcd versions.

etcd 3.0 to 3.1 upgrade path: Continue to use the etcd_grpc_requests_total. Once the upgrade is complete and all etcd nodes are running 3.1, migrate to the grpc_server_handled_total metric at your leisure.

This PR reorganizes the code substantially. Previously, the code to proxy etcd metrics was hard coded and limited to a single counter metric. This has been entirely replaced with code that generically filters, rewrites proxied etcd metrics and then aggregates them with custom metrics such as the etcd version metric.

cc @wojtek-t @mml @shyamjvs @cheftako

Fix `etcd-version-monitor` to backward compatibly support etcd 3.1 [go-grpc-prometheus](https://github.com/grpc-ecosystem/go-grpc-prometheus) metrics format.