Metrics feature flag for `light-client` REST APIs

### Problem description

In order to make a proposal for inclusion of the light-client to browser extensions such as Metamask, we need to demonstrate the resourcefulness of running the Lodestar `light-client` in a browser environment. We would need to implement this to enable REST APIs for metrics collection.

### Solution description

We would hide this ability through a feature flag defaulted as `false` and include some metrics for collection such as:
- Memory usage
- CPU usage
- Response times
- BLS related metrics for browser

### Additional context

As discussed in our mini-chat with Greg about demonstrating the performance of our light client to browser based wallets such as Metamask.