Add some metrics to debuginfod and use them to make test sleep less

[Mark Wielaard <mark at klomp dot org>]

Hi,

The following two patches add some metrics to debuginfod that can be
used ito see whether the scanners are still finding new executables,
debuginfo files or find new sources. They can also be used in the
run-debuginfod-find.sh testcase to see whether the server is ready
or not for the next test. This removes most sleeps from the testcase
(and the test_webapi_sleep in the server).

[1/2] debuginfod: Add found_{executable,debuginfo,sourcerefs}_total metrics.
[2/2] tests: Don't sleep in run-debuginfod-find.sh, but wait till ready.

This brings down the run-debuginfod-find.sh run from 2 minutes and 10
seconds to 8 seconds on my machine. And from 3 minutes to 45 seconds
under valgrind

It doesn't actually test that the metrics are correctly set (even
though it calculates what the metrics should be) because the metrics
are not guaranteed to be stable. But maybe the test should?
It could just fail instead of timeing out. Then when we change the
metrics we could just update the test simultaniously?

Cheers,

Mark