Metrics feature flag for `light-client` REST APIs

[philknows]

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.