feat: Add namespace and subsystem configuration

[phbernardes]

• Accept namespace and subsystem parameters in instrument definition.

Hi, I open this PR to be able to set the namespace and subsystem during instrumentation initialisation.

Having this parameter is important for projects where several metrics endpoints are fetched and metrics are squashed together.

This will allow us to have the same behaviour as prometheus-flask-instrumentator (when using the defaults_prefix argument of PrometheusMetrics).

Thank you.

[M3g4lodon]

This should fulfill our needs for our app in production!

[phbernardes]

@trallnag do you think this PR can be merged?

[codecov]

Codecov Report

Merging #193 (fd8dd90) into master (e545785) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #193   +/-   ##
=======================================
  Coverage   91.07%   91.07%           
=======================================
  Files           5        5           
  Lines         280      280           
=======================================
  Hits          255      255           
  Misses         25       25           

Impacted Files	Coverage Δ
...ometheus_fastapi_instrumentator/instrumentation.py	90.90% <100.00%> (ø)
...rc/prometheus_fastapi_instrumentator/middleware.py	88.15% <100.00%> (ø)

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[trallnag]

Thank you, @phbernardes