Group servers in upstream

[Goodluckhf]

This PR fix issue: If you put the same server multiple times to upstream you will get an error "...was collected before with the same name and label" But I have to use this for retries to the same server

[KostyaEsmukov]

upstream.ResponseMsec seems to be computed as an average, but computing an average on top of averages cannot technically be considered an average. For example, I think for a single server in upstream with ResponseMsec=42ms the aggregated ResponseMsec would be 21ms ((0 + 42) / 2 = 21), which is different from the expected 42ms.

I'd convert the averages back to the sums of times (assuming that RequestCounter is the denominator of the averages), and then compute the aggregated average using the sums, something like this:

if (groupedByServer[index].RequestCounter + upstream.RequestCounter) > 0 {
	groupedByServer[index].RequestMsec = (groupedByServer[index].RequestMsec * groupedByServer[index].RequestCounter + upstream.RequestMsec * upstream.RequestCounter) / (groupedByServer[index].RequestCounter + upstream.RequestCounter)
	groupedByServer[index].ResponseMsec = (groupedByServer[index].ResponseMsec * groupedByServer[index].RequestCounter + upstream.ResponseMsec * upstream.RequestCounter) / (groupedByServer[index].RequestCounter + upstream.RequestCounter)
}
groupedByServer[index].RequestCounter += upstream.RequestCounter  // note that this increment has to happen *after* the averages

Despite this little issue -- thank you for the great patch!

(Note, I'm not a maintainer of this project, I just faced the same issue and have reused your helpful solution).