Ability to disable iOS & Android user agent check in HCM

*Title*: Ability to disable the user-agent check for iOS and Android

*Description*:
Hi, while checking for metrics to stop emitting, I discovered the user-agent metric that is being emitted.
From a strictly service-to-service standpoint, I don't see any usage for this `user-agent` metric, so would like to disable this in the HCM, but I see no option to.

I believe a controllable option on whether to have the `user-agents` checked or not in the HCM may possibly help performance?
https://github.com/envoyproxy/envoy/blob/main/source/common/http/user_agent.cc#L18

Thanks