Feature request: add AppMetricsBeforeMiddleware.label_metric()

[wilsoniya]

Background

At present, metrics emitted by AppMetricsBeforeMiddleware are not labeled and no hook exists to apply ad-hoc labels to these metrics; see:

django-prometheus/django_prometheus/tests/end2end/testapp/test_middleware_custom_labels.py

Lines 19 to 46 in a304687

	class CustomMetrics(Metrics):
	def register_metric(self, metric_cls, name, documentation, labelnames=(), **kwargs):
	if name in EXTENDED_METRICS:
	labelnames.extend(("view_type", "user_agent_type"))
	return super().register_metric(
	metric_cls, name, documentation, labelnames=labelnames, **kwargs
	)
	class AppMetricsBeforeMiddleware(PrometheusBeforeMiddleware):
	metrics_cls = CustomMetrics
	class AppMetricsAfterMiddleware(PrometheusAfterMiddleware):
	metrics_cls = CustomMetrics
	def label_metric(self, metric, request, response=None, **labels):
	new_labels = labels
	if metric._name in EXTENDED_METRICS:
	new_labels = {"view_type": "foo", "user_agent_type": "browser"}
	new_labels.update(labels)
	return super().label_metric(metric, request, response=response, **new_labels)
	@override_settings(
	MIDDLEWARE=get_middleware(
	"testapp.test_middleware_custom_labels.AppMetricsBeforeMiddleware",

Proposal

Add a label_metric() method to django_prometheus.middleware.AppMetricsBeforeMiddleware which would function identically to django_prometheus.middleware.AppMetricsAfterMiddleware.label_metric(), which already exists.

Use case

The ability to apply labels unconditionally to all emitted metrics. For example, applying the namespace label allows the django_prometheus grafana dashboard to query on a namespace label: https://grafana.com/grafana/dashboards/9528-django-prometheus/

📣 If the authors are amenable to this change, I'm happy to implement it and open a PR