solana: 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`