Document falsey values of job_metrics

doc/source/admin/galaxy_options.rst

@@ -4309,6 +4309,8 @@
     The syntax, available instrumenters, and documentation of their
     options is explained in detail in the documentation:
     https://docs.galaxyproject.org/en/master/admin/job_metrics.html
+    By default, the core plugin is enabled. Setting this option to
+    false or an empty list disables metrics entirely.
 :Default: ``None``
 :Type: seq
 

lib/galaxy/config/sample/galaxy.yml.sample

@@ -2337,6 +2337,8 @@ galaxy:
   # The syntax, available instrumenters, and documentation of their
   # options is explained in detail in the documentation:
   # https://docs.galaxyproject.org/en/master/admin/job_metrics.html
+  # By default, the core plugin is enabled. Setting this option to false
+  # or an empty list disables metrics entirely.
   #job_metrics: null
 
   # This option allows users to see the job metrics (except for

lib/galaxy/config/schemas/config_schema.yml

@@ -3145,6 +3145,9 @@ mapping:
 
           https://docs.galaxyproject.org/en/master/admin/job_metrics.html
 
+          By default, the core plugin is enabled. Setting this option to false or an empty list disables metrics
+          entirely.
+
       expose_potentially_sensitive_job_metrics:
         type: bool
         default: false

lib/galaxy/job_metrics/__init__.py

@@ -79,6 +79,7 @@ def __init__(self, conf_file=None, conf_dict=None, **kwargs):
                 conf_dict = DEFAULT_CONFIG
             self.default_job_instrumenter = JobInstrumenter.from_dict(self.plugin_classes, conf_dict, **kwargs)
         else:
+            # allows for setting non-None falsey values to get no metrics config whatsoever
             self.default_job_instrumenter = NULL_JOB_INSTRUMENTER
         self.job_instrumenters = collections.defaultdict(lambda: self.default_job_instrumenter)