Add a method of disabling prometheus & make it efficient

Gathering metrics can have a non-negligible cost in time or memory,
so make it possible to disable. Make the disabled case avoid
installation of a callback.

docs/user-guide.rst

@@ -240,7 +240,8 @@ Prometheus metrics
 
 This library automatically collects `Prometheus <https://prometheus.io/>`_
 metrics if the ``prometheus_client`` Python module is available.
-The feature is disabled when this module is not installed.
+The feature is disabled when this module is not installed or if the
+``MORE_EXECUTORS_PROMETHEUS`` environment variable is set to ``0``.
 
 If you want to ensure that ``more-executors`` is installed along with
 all prometheus dependencies, you may request the 'prometheus' extras,

more_executors/_impl/metrics/__init__.py

@@ -1,3 +1,4 @@
+import os
 import logging
 from functools import partial
 
@@ -7,14 +8,17 @@
 
 LOG = logging.getLogger("more-executors.metrics")
 
-try:  # pylint: disable=import-error
-    from .prometheus import PrometheusMetrics
+if os.environ.get("MORE_EXECUTORS_PROMETHEUS", "1") == "0":
+    metrics = NullMetrics()  # type: ignore
+else:
+    try:  # pylint: disable=import-error
+        from .prometheus import PrometheusMetrics
 
-    metrics = PrometheusMetrics()
-except Exception:
-    LOG.debug("disabling prometheus support", exc_info=True)
+        metrics = PrometheusMetrics()
+    except Exception:
+        LOG.debug("disabling prometheus support", exc_info=True)
 
-    metrics = NullMetrics()  # type: ignore
+        metrics = NullMetrics()  # type: ignore
 
 
 def record_done(f, started_when, time, inprogress, cancelled, failed):
@@ -60,3 +64,11 @@ def track_future(f, **labels):
     f.add_done_callback(cb)
 
     return f
+
+
+def track_future_noop(f, **_labels):
+    return f
+
+
+if isinstance(metrics, NullMetrics):
+    track_future = track_future_noop