Client libraries MUST NOT allow users to have different label names for the same 
metric for Gauge/Counter/Summary/Histogram or any other Collector offered by the 
library.

# initialize metric 'foo' and cache it
$m = getStatsFactory()->getCounter('foo')->setLabel('bar', 'a');
$m->increment(); # MediaWiki_foo{bar=a} 1

# now fetch same metric from cache, but forget to set the label
$m = getStatsFactory()->getCounter('foo');
$m->getLabelKeys(); # ['bar']
$m->increment();  # MediaWiki_foo{} 1

# now set a new label on the same metric
# this is caught by setLabel() disallowing adding labels when samples are recorded
$m = getStatsFactory()->getCounter('foo')->setLabel('baz', 'a'); # NullMetric
$m->increment();  # no effect