Identifies a metric, by describing the source which generated
Origin (namespace) of metric name. May be
blank for user-define metrics; will be
"dataflow" for metrics defined by the Dataflow
service or SDK.
Worker-defined metric name.
Zero or more labeled fields which identify the part of the
job this metric is associated with, such as the name of a
step or collection.
For example, built-in counters associated with steps will
have context['step'] = . Counters associated with
PCollections in the SDK will have context['pcollection'] = .