Example on how to expose metrics in a flask app

[banjoh]

I'm working on a flask application that needs to expose metrics to prometheus. It was not very clear how that could be done without reading the code. This PR updates the README with a simple example of doing so. Hopefully it saves someone some time of trying to figure that out.

@brian-brazil

[brian-brazil]

This doesn't pull in all the other logic and endpoint should have. Can you reuse one of the ones that handles this for you?

[brian-brazil]

I don't want this logic duplicated here, I want you to use one of the existing functions for it. This code will change over time, and copy&pasted code in user applications won't get updated.

[banjoh]

Do you mean using like so?

from prometheus_client import REGISTRY
def metrics():
    r = REGISTRY

If not, I'm probably missing the point. In my initial implementation I was hoping/expecting to find a clean function that returns a response having parsed out the parameters and done its magic. A new public api could be added that does something like this

def generate_latest(registry=core.REGISTRY, query_string=None)
    """Generate the latest metrics taking the query string into account"""
    metrics_subset = parse_qs(query_string).get('name[]') if query_string is not None else None
    if metrics_subset:
        r = r.restricted_registry(metrics_subset)
    ...

[brian-brazil]

No, I mean reusing something like MetricsHandler that does all the http stuff.

[banjoh]

Since flask's response is a uwsgi application, I'll try to see how I can reuse make_wsgi_app as is.

[banjoh]

@brian-brazil I think the example should be good to go now.

[brian-brazil]

Flask

[brian-brazil]

Wsgi should stay first

[brian-brazil]

Keep this minimal, just enough to expose metrics at all like in the other examples.

[banjoh]

Done

[brian-brazil]

Flask Prometheus WSGI

[banjoh]

Not sure I understand this comment

[brian-brazil]

Proper nouns should be capitalised.

[banjoh]

Done

[brian-brazil]

We mention filenames before the relevant example.

[banjoh]

Done

[brian-brazil]

s/Promethues//

[banjoh]

Oops :) Typo fixed

[brian-brazil]

Thanks!