Custom Collectors

Sometimes it is not possible to directly instrument code, as it is not in your control. This requires you to proxy metrics from other systems.

To do so you need to create a custom collector, for example:

from prometheus_client.core import GaugeMetricFamily, CounterMetricFamily, REGISTRY
from prometheus_client.registry import Collector

class CustomCollector(Collector):
    def collect(self):
        yield GaugeMetricFamily('my_gauge', 'Help text', value=7)
        c = CounterMetricFamily('my_counter_total', 'Help text', labels=['foo'])
        c.add_metric(['bar'], 1.7)
        c.add_metric(['baz'], 3.8)
        yield c

REGISTRY.register(CustomCollector())

SummaryMetricFamily, HistogramMetricFamily and InfoMetricFamily work similarly.

A collector may implement a describe method which returns metrics in the same format as collect (though you don’t have to include the samples). This is used to predetermine the names of time series a CollectorRegistry exposes and thus to detect collisions and duplicate registrations.

Usually custom collectors do not have to implement describe. If describe is not implemented and the CollectorRegistry was created with auto_describe=True (which is the case for the default registry) then collect will be called at registration time instead of describe. If this could cause problems, either implement a proper describe, or if that’s not practical have describe return an empty list.