No error on missing labels for Gauge.set_function()

[pietrodn]

If I try to assign a function to a Gauge using set_function(), the Prometheus library does not error even if the labels are missing.

This code should error, but it does not:

import prometheus_client

processed_input_messages = prometheus_client.Gauge(
    name="my_gauge",
    documentation="A generic Gauge.",
    labelnames=["a", "b"],
)

if __name__ == '__main__':
    # This should fail, because the "a" and "b" labels are not provided!
    processed_input_messages.set_function(lambda: 1)

In general, it would be important to check, for every metric type, if all the labels are set when calling .set(), .inc(), .set_function(), and the other similar metrics.