`--metrics` does not expose span histogram metrics unless `--tracing` is also enabled

## Summary

The `--metrics` flag only exposes a small subset of metrics unless `--tracing` is also turned on. In our production deployment, with metrics enabled, we only see:
- `rollup_boost_execution_mode`
- `rollup_boost_rpc_blocks_created`

The docs suggest that enabling `--metrics` should also expose span duration histograms such as:

- `fork_choice_updated_v3_duration`
- `get_payload_v3_duration`
- `new_payload_v3_duration`

However, those histogram metrics appear to be gated behind `--tracing`, not `--metrics` ([code ref](https://github.com/flashbots/rollup-boost/blob/main/crates/rollup-boost/src/tracing.rs#L150)).

## Suggested fix

I think the cleanest fix would be to decouple Prometheus span metrics from OTLP tracing export.

Read crates/rollup-boost/src/tracing.rs around line 150 and trace how the --metrics and --tracing flags control span metrics. Done means the listed span duration histograms are exposed with --metrics alone, while OTLP tracing export remains independently controlled.

Written by the indexing model from the issue text.