Add a clear() method to metric objects to remove all labelsets

[Veknelyon]

Hello @brian-brazil,

I wanted to test metric values in my application unit tests but I didn't figured out how to do it (I probably missed something). That's why I propose to add this simple reset() method. Do you think it can be useful? Or is there a better way to test than reseting metrics at the beginning of the test?

And thank you for this very useful lib!

[csmarchbanks]

Hello, thank you for the contribution!

Typically the best way to test metrics is to create a separate registry for each test for the best isolation, or to unregister the metric from a shared registry at the end of the test. Would one of those methods work for your use case?

[Veknelyon]

It does not work in my case because my metrics are global variables shared in my code. So changing the registry does not reset values stored in the metric.
Here is a simplification of my code:

from prometheus_client import Counter, CollectorRegistry, REGISTRY
from pytest import fixture

RUN_COUNTER = Counter('connector_run', labelnames=['tenant'])


def my_function():
    RUN_COUNTER.labels('tenantid').inc()
    # do things


@fixture
def metric_registry():
    registry = CollectorRegistry()
    registry.register(RUN_COUNTER)
    return registry


def test1(metric_registry):
    my_function()
    
    # other asserts


def test2(metric_registry):
    my_function()

    run_count = metric_registry.get_sample_value('connector_run_total', labels={'tenant': 'tenantid'})
    assert run_count == 1.0

(the code is not as simple so I need to keep test1 and test2 separated)

When I run this code assert run_count == 1.0 failed because test1 is executed first (and I can't presume test execution order).

My issue come from the fact that this is not the registry which store the metric value but RUN_COUNTER which is global.
With the reset() method, I would be able to write:

@fixture
def metric_registry():
    RUN_COUNTER.reset()
    registry = CollectorRegistry()
    registry.register(RUN_COUNTER)
    return registry

[csmarchbanks]

That makes sense thanks! I left one question/suggestion, but generally I am happy to add a function like this.

[csmarchbanks]

What would you think of naming this remove_all instead of reset to match remove? I could imagine reset also meaning something like "reset all values to zero" or something else.

[csmarchbanks]

Actually, according to https://prometheus.io/docs/instrumenting/writing_clientlibs/#labels, I think it should be called clear().

[Veknelyon]

If the documentation say so. I'll rename it clear.

[csmarchbanks]

Thanks!