feat: record validation and end-to-end response time in signer metrics

[hstove]

• Closes Add signer prometheus metrics for block validation time #5588

This PR adds two metrics:

• stacks_signer_block_response_latencies_histogram: this measures the "end-to-end" time that the signer takes to respond to a block proposal. The block's timestamp is used as the "start time", so this isn't a totally fair end-to-end measurement, but it's directionally accurate.
• stacks_signer_block_validation_latencies_histogram: the validation_time_ms reported by the node when validating a block proposal

[obycode]

No strong preference here, but I've seen this pattern instead of allowing unused_variables:

pub fn record_block_validation_latency(latency_ms: u64) {
    #[cfg(not(feature = "monitoring_prom"))]
    let _ = latency_ms; // This prevents unused variable warnings

    #[cfg(feature = "monitoring_prom")]
    {
        ...
    }
}

[obycode]

LGTM. I just left one optional comment. Feel free to ignore it.

[rdeioris]

LGTM