[APM] Cancel requests (both server-side and browser-side)

[dgieselaar]

We've seen examples of a cluster becoming overwhelmed, or at least search tasks getting queued, when a search is slow and the user starts clicking around in the UI. One possible cause is that we don't cancel running searches, and ES starts queuing them up, resulting in an unresponsive UI (at least in terms of data).

ES will cancel a search when a connection closes. We can leverage this mechanism if we cancel requests browser side, and close the connection to ES when the browser request is closed.

We can look at the search strategy service for an example of how to do this.

[elasticmachine]

Pinging @elastic/apm-ui (Team:apm)

[sorenlouv]

@dgieselaar wouldn't this be handled for us if we used the kibana search service?

[dgieselaar]

Canceling the request server-side will be handled by #86594. Canceling the request browser-side, e.g. when a component unmounts or when a new request starts is a little more complicated: useFetcher know when a component mounts/unmounts, but does not know anything about callApmApi. It's fixable but maybe some more work, I suggest we do that separately.