Allow setting counter value when exposing existing counts

I am just started to use prometheus, and your library and have come across this issue.

Counter only allows you to increase the value which is fine if you actually using it to count things but breaks down if you have an existing counter you are trying to expose though prometheus.

E.g. the DotNetStatsCollector seems to increment based on a calculated diff. _cpuTotal.Inc(_process.TotalProcessorTime.TotalSeconds - _cpuTotal.Value); This looks like a race condition to me as I cannot see anything stopping two scrapes occurring at the same time. In fact as I understand it multiple parallel scrapes is the preferred way to achive high availability of the prometheus server.

[gambrose]

@sandersaares I took a look at the go client (I do not use go so I could be wrong). They had a set method on counter but it was removed in favour of using a ConstantMetric that is just used when creating a custom collector.

https://github.com/prometheus/client_golang/blob/b49b54cdb5f9850eb4c0090e5f9a8b3fc5113d5d/prometheus/value.go#L81

[godefroi]

I have no experience with any other Prometheus client libraries. I think that #73 expresses a sufficient resolution; if it were clear how to create a custom class that could report to Prometheus that it was a counter, but in fact behave from an API perspective as a gauge, then I would be satisfied. Windows performance counters are the obvious use case here, but it’s quite possible that there are others. While I would love to rewrite all my software to expose metrics directly instead of relying on performance counters, in reality I have >150 environments all running various versions of my products, and updating each version to expose metrics to Prometheus is a gargantuan task that will realistically take years. I would like to use Prometheus as best I can in the mean time.