Use ring client pool in ruler to reduce number of gRPC connections created

[pracucci]

Is your feature request related to a problem? Please describe.
When sharding is enabled, the ruler connects to all other rulers to retrieve all rules (see Ruler.getShardedRules()). To do it, we open a gRPC connection towards each ruler, each time the getShardedRules() is called. This leads to a bunch of new TCP connections created on a high traffic cluster.

Describe the solution you'd like
Instead of opening a gRPC connection each time, we should use the ring's client.NewPool() to keep connections open and reuse clients.

[stale]

This issue has been automatically marked as stale because it has not had any activity in the past 60 days. It will be closed in 15 days if no further activity occurs. Thank you for your contributions.

[jtlisi]

still valid

[jtlisi]

still valid

[stale]

This issue has been automatically marked as stale because it has not had any activity in the past 60 days. It will be closed in 15 days if no further activity occurs. Thank you for your contributions.

[pracucci]

Done in #3916.