[Feature] Add new metric to capture client store connection.

[hsuttong]

Willingness to contribute

Yes. I can contribute a fix for this bug independently.

Feature Request Proposal

Add a new metric to track store discovery requests.

Motivation

What is the use case for this feature?

To build a complete signal for determining if a Venice store is no-longer in use we need a new metric that captures a clients connection to a store. Along with this new metric we can combine read, write, store age metrics to create a signal for unused stores.

Details

No response

What component(s) does this bug affect?

• Controller: This is the control-plane for Venice. Used to create/update/query stores and their metadata.
• Router: This is the stateless query-routing layer for serving read requests.
• Server: This is the component that persists all the store data.
• VenicePushJob: This is the component that pushes derived data from Hadoop to Venice backend.
• VenicePulsarSink: This is a Sink connector for Apache Pulsar that pushes data from Pulsar into Venice.
• Thin Client: This is a stateless client users use to query Venice Router for reading store data.
• Fast Client: This is a stateful client users use to query Venice Server for reading store data.
• Da Vinci Client: This is an embedded, stateful client that materializes store data locally.
• Samza: This is the library users use to make nearline updates to store data.
• Admin Tool: This is the stand-alone client used for ad-hoc operations on Venice.