The request subscriber writes to state on every request of the site

Problem/Motivation

MyrestResourceResponseSubscriber::onRequest() subscribes to KernelEvents::REQUEST at priority 10, so it runs for every request the site serves. When count_load.enable is on it reads myrest_count_load from state, updates three counters and writes state back — a read and a write to the shared key-value store on every page view, HTML pages and API requests alike. Two problems follow: a module installed for its API is now in the critical path of every page of the site, and the counter is written non-atomically, so concurrent requests overwrite each other's increments and the number is wrong anyway. It also sets a request attribute on every request whether or not anything reads it.

Proposed resolution

• Count only this module's own requests, which is what the counter is for — it already distinguishes them for its count_api figure.
• Replace the read-modify-write with something that survives concurrency, or state plainly in the interface that the figure is approximate.
• Set the timing attribute only when the timing display is enabled.
• Assert with a test that a non-API request does no state write, which is the guarantee this issue is really about.

Remaining tasks

Everything. Found while auditing what the module changes outside its own endpoints.