rec: generate metrics files (also) where meson expects them

[omoerbeek]

This avoids unnecessary regeneration of files. Generating into the src dir as we do is a bit rough, but the moment we drop autotools it will not be needed anymore.

Short description

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)
• checked that this code was merged to master

[coveralls]

Pull Request Test Coverage Report for Build 12252966312

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 812 unchanged lines in 11 files lost coverage.
• Overall coverage increased (+0.07%) to 64.759%

---

Files with Coverage Reduction	New Missed Lines	%
pdns/misc.cc	1	64.62%
ext/json11/json11.cpp	1	64.49%
pdns/recursordist/sortlist.cc	2	72.94%
pdns/rcpgenerator.cc	2	90.1%
pdns/recursordist/recpacketcache.hh	3	89.55%
pdns/recursordist/test-syncres_cc1.cc	7	89.18%
modules/lmdbbackend/lmdbbackend.cc	14	72.54%
pdns/tcpiohandler.cc	17	68.18%
pdns/recursordist/test-syncres_cc10.cc	31	89.59%
pdns/recursordist/syncres.cc	141	79.37%

Totals
Change from base Build 12230339803:	0.07%
Covered Lines:	125901
Relevant Lines:	163534

---

💛 - Coveralls

[Habbie]

but the moment we drop autotools it will not be needed anymore.

maybe leave a FIXME note for that