Add metric tracking the time spent inside gateway

Is your feature request related to a problem? Please describe.
No. Feature is not related to a problem

Describe the solution you'd like
In the same vain as spring.cloud.gateway.requests it would be useful to have a metric like spring.cloud.gateway.internal.latency (example, not necessarily real name) tracking the time spend inside the gateway.
It would have the same kind of tags as spring.cloud.gateway.requests to be able to monitor the latency based on criterion.
Ex:
For a 100 ms round trip spring.cloud.gateway.requests , spring.cloud.gateway.requests would report 3ms. Meaning that gateway counted for 3% of the total time.
We do not have the capability to determine gateway latency and therefore cannot measure the impact of changes or improvements in our security or caching inside the gateway.

Describe alternatives you've considered
We considered developing our own metric collection inside that gateway but we're not familiar (tbh competent would be a better word).

Additional context
Versions

• spring-cloud-gateway: 3.1.0
• java 11

Start by locating the existing spring.cloud.gateway.requests metric instrumentation and its tags. Determine where gateway-internal time can be measured separately from round-trip time, then verify that the new metric reports gateway latency with equivalent tags and distinguishes it from total request duration.