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

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### 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.
- [X] `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.

Start with the Router component and trace how store discovery requests are handled and currently measured. Determine the metric's scope, naming, labels, and reporting behavior from existing Router conventions; done means the new connection metric supports the stated unused-store signal and has coverage for its expected behavior.

Written by the indexing model from the issue text.