refactor: put metric collection in helper methods

[JohannesMessner]

Pure refactoring PR, pulling the metrics collection for Prometheus into separate methods.

[samsja]

lgtm lets wait for the CI to pass I suggest a renaming

[samsja]

lgtm, great refactoring

[codecov]

Codecov Report

Merging #5337 (f1b6077) into master (bd80035) will increase coverage by 1.34%.
The diff coverage is 96.42%.

@@            Coverage Diff             @@
##           master    #5337      +/-   ##
==========================================
+ Coverage   85.27%   86.62%   +1.34%     
==========================================
  Files          97       99       +2     
  Lines        6413     6468      +55     
==========================================
+ Hits         5469     5603     +134     
+ Misses        944      865      -79     

Flag	Coverage Δ
jina	86.62% <96.42%> (+1.34%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...untimes/request_handlers/worker_request_handler.py	97.50% <96.42%> (+0.14%)	⬆️
jina/serve/instrumentation/_aio_server.py	77.14% <0.00%> (-11.43%)	⬇️
jina/serve/runtimes/gateway/websocket/gateway.py	98.21% <0.00%> (-1.79%)	⬇️
jina/resources/health_check/gateway.py	41.17% <0.00%> (ø)
jina/resources/health_check/pod.py	50.00% <0.00%> (ø)
jina/orchestrate/deployments/config/k8s.py	100.00% <0.00%> (+0.61%)	⬆️
jina/serve/networking.py	87.03% <0.00%> (+0.82%)	⬆️
jina/serve/runtimes/asyncio.py	85.45% <0.00%> (+0.90%)	⬆️
jina/clients/base/websocket.py	90.19% <0.00%> (+0.98%)	⬆️
jina/clients/base/grpc.py	85.91% <0.00%> (+1.40%)	⬆️
... and 14 more

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[samsja]

lgtm