Allow setting value of counter metrics

[leedm777]

I have the need for counter metrics that are set explicitly, instead of being incremented. I don't have access to the system tracking the metric, I only have whatever the current value is. One good example of this is to provide an uptime metric.

$u = getCurrentUptime();
$uptime = $registry->registerCounter($ns, 'uptime_seconds_total', 'Current uptime in seconds');
$uptime->set($u);

[bracki]

You wouldn't use a Counter but rather a Gauge for that type of thing.

$gauge = $registry->getOrRegisterGauge('test', 'some_gauge', 'it sets', ['type']);
$gauge->set(2.5, ['blue']);