Include skip_methods option to avoid collecting metrics for specific methods

[adinsoon]

What does this Pull Request do?

Adds an option to avoid collecting metrics on request with specific methods.
This option can be used to avoid collecting metrics on, for example, communication description requests for endpoints (but can be used in any other method).
The new option is an optional list called skip_methods.

Example:

app.add_middleware(
  PrometheusMiddleware,
  app_name="hello_world",
  prefix='myapp',
  labels={
      "server_name": os.getenv("HOSTNAME"),
  }),
  group_paths=True,
  buckets=[0.1, 0.25, 0.5],
  skip_paths=['/health'],
  skip_methods=['OPTIONS'],
  always_use_int_status=False),
  exemplars=lambda: {"trace_id": get_trace_id}  # function that returns a trace id

On the justification

We have an application using microservices architecture, at the beginning of which there is a gateway that forwards requests with a given path or method. From this point, we monitor the metrics. Prometheus is not able to process such a large generated traffic, which we already reduce by ignoring some paths, so we also want to reduce it by ignoring requests containing some methods (namely OPTIONS), because they are not important to us. This change may allow us to get a performance benefit. I will be very pleased with the opportunity to contribute to your project.

@stephenhillier

[stephenhillier]

@adinsoon thanks! This looks totally reasonable. I am away for the next couple days and will look at merging this and cutting a new release soon. Thank you for your patience.

[stephenhillier]

Going to bump the minor version to 0.16.0 to indicate there's a new feature here. I'll take care of it when I create the release though, just letting you know.

[stephenhillier]

Looks great, thank you for the contribution! Thanks for updating the docs and adding a test too. Cheers

[adinsoon]

@stephenhillier Great that it came in handy, thank you for merging, take care