Decouple metrics submission from RUST_LOG

Problem

It is occasionally desired to reduce text log size, but continue submitting full metrics. With metrics level taken from RUST_LOG, this is not possible.

Relevant Code

Datapoints are pushed to the MetricsAgent based on log level here: https://github.com/solana-labs/solana/blob/d4ce59eee7e2537f02c827b6918de9af559b06bd/metrics/src/datapoint.rs#L153-L165

If the datapoint has sufficient log-level based on RUST_LOG to get submitted, then it will eventually reach here where it is both logged to file and pushed into a vector to be sumitted. https://github.com/solana-labs/solana/blob/d4ce59eee7e2537f02c827b6918de9af559b06bd/metrics/src/metrics.rs#L261-L264

Proposed Solution

Look into the feasibility of decoupling metrics from RUST_LOG