Use a gauge for the histogram sum

[Neverlord]

The online docs for Prometheus state:

The sum of observations (...) behaves like a counter, too, as long as
there are no negative observations.

A counter can only ever go up and asserts that users pass in only positive values when incrementing. Hence, the sum must use a gauge in order to stay consistent with the Prometheus documentation.

Source for the quote: https://prometheus.io/docs/practices/histograms/#count-and-sum-of-observations

[Neverlord]

Btw, I don't think I can do something about the failed coverage check:

2020-07-03T16:44:58.5410598Z Traceback (most recent call last):
2020-07-03T16:44:58.5411044Z   File "/home/runner/.local/bin/coveralls", line 11, in <module>
2020-07-03T16:44:58.5411274Z     sys.exit(run())
2020-07-03T16:44:58.5412163Z   File "/home/runner/.local/lib/python2.7/site-packages/cpp_coveralls/__init__.py", line 92, in run
2020-07-03T16:44:58.5412546Z     raise ValueError("\nno coveralls.io token specified and no travis job id found\n"
2020-07-03T16:44:58.5413184Z ValueError: 
2020-07-03T16:44:58.5413702Z no coveralls.io token specified and no travis job id found
2020-07-03T16:44:58.5414335Z see --help for examples on how to specify a token
2020-07-03T16:44:58.5414539Z 
2020-07-03T16:44:58.5568476Z ##[error]Process completed with exit code 1.

[jupp0r]

Would be nice if you could add a unit test to verify this works as expected.

[dota17]

I think the failed on coverage check is strange, there is nothing about travis job id, but log said it failed to find this.

btw, I also think the file range selected by Coverage is wrong:

Total Test time (real) =  20.39 sec
File '/home/runner/work/prometheus-cpp/prometheus-cpp/core/tests/gauge_test.cc'
Lines executed:100.00% of 63
Creating '#home#runner#work#prometheus-cpp#prometheus-cpp#_build_coverage#core#tests#CMakeFiles#prometheus_core_test.dir#gauge_test.cc.o###home#runner#work#prometheus-cpp#prometheus-cpp#core#tests#gauge_test.cc.gcov'

File '/usr/local/share/vcpkg/installed/x64-linux/include/gtest/internal/gtest-internal.h'
Lines executed:8.70% of 23
Creating '#home#runner#work#prometheus-cpp#prometheus-cpp#_build_coverage#core#tests#CMakeFiles#prometheus_core_test.dir#gauge_test.cc.o###usr#local#share#vcpkg#installed#x64-linux#include#gtest#internal#gtest-internal.h.gcov'

File '/usr/include/c++/5/iostream'
Lines executed:100.00% of 1
Creating '#home#runner#work#prometheus-cpp#prometheus-cpp#_build_coverage#core#tests#CMakeFiles#prometheus_core_test.dir#gauge_test.cc.o###usr#include#c++#5#iostream.gcov'

If you like, I'd love to make a little patch to fix this.

[Neverlord]

Would be nice if you could add a unit test to verify this works as expected.

Will do. 🙂

[gjasny]

Closing in favour of #406.