Add PHP-FPM memory peak to scoreboard

[flavioheleno]

This PR adds a new entry to PHP-FPM's scoreboard to track peak memory usage.

This is useful to track memory usage and help better tune PHP/PHP-FPM configuration.

Performance impact should be minimal as it is only updated in request end (fpm_request_end).

[bukka]

You might want to read: #10600 (comment) . TLDR not sure if this is really useful as it just reports peak of the last request...

[flavioheleno]

The idea behind this change is to record the largest peak in memory usage between requests.

From what I can understand from the code, the fpm_request_end function is executed for all requests and the treatment done by fpm_scoreboard_update_commit is what guarantees the recording of the largest peak (similar to the treatment done for active max), or am I wrong?

In general, we now have a better view of memory consumption, regardless of how often the extractor consumes data (as is the case when we use the data provided by full, which only provides data for the last request).

[bukka]

I had a better look now and after thinking about it I think it actually makes sense to add it. We have got already some similar fields and I see the use case here. I will try to do a bit more testing but it should make it to 8.4