The presto_server_prepare_response_time metric doesn't show all paths

[jakubgs]

Describe the bug
The presto_server_prepare_response_time metrics shows only some paths, not all:

jakubgs@geth-01.ih-eu-mda1.nimbus.hoodi:~ % c 0:9203/metrics | grep presto_server_prepare_response_time
# HELP presto_server_prepare_response_time Time taken to prepare response
# TYPE presto_server_prepare_response_time gauge
presto_server_prepare_response_time{endpoint="GET/eth/v1/beacon/states/{state_id}/pending_deposits"} 717.0
presto_server_prepare_response_time{endpoint="GET/eth/v1/beacon/states/{state_id}/validators/{validator_id}"} 676.0

One path it does not list is /eth/v1/validator/register_validator.

Additional context
It would be actually better if this metric(or a new one) was using buckets to provide this information, so it's possible to graph it as a heatmap.