feat(rpc/validation): Expose metric for validation disallow list size.

[ryanschneider]

For compliance reasons it's desirable to have a metric for the validation API's disallow list size (for example an infra provider can setup an alert on this metric if it's 0 or lower than expected as it might indicate the disallow list is misconfigured).

Example metric output:

# HELP reth_builder_validation_disallow_size The number of entries configured in the builder validation disallow list
# TYPE reth_builder_validation_disallow_size gauge
reth_builder_validation_disallow_size 1

[mattsse]

this just adds an additional metric so even if this is never changed it should be fine

[mattsse]

can we add a smol note here why this exists

[mattsse]

I'd prefer a oneliner here with inner.disallwo.len()