I'd like to measure avg request-response time for my webserver.
Apm has transaction.duration.us and it seems this could be the metric I'm looking for.
But I coulnd't find the documentation what it is.
Where can I find the meaning of the variable?
transaction.duration.us should indeed be what you're looking for. It's the duration in microseconds as an integer. Divide it by 1000 to get milliseconds, or by 1'000'000 to get seconds.
https://www.elastic.co/guide/en/apm/server/7.9/exported-fields-apm-transaction.html#_duration_2