Data race causing incorrect values for solr_up metric

[Izbitzer]

Running the following simple bash script causes the first request to get a value of 0 for the solr_up metric:

curl -s localhost:9231/metrics &                                                                                        
curl -s  localhost:9231/metrics &  

It appears to be a data race, as changing line 228 of exporter.go to the following fixes it:

defer func() {
    ch <- e.up
    time.Sleep(1 * time.Millisecond)
}()

I don't know Go well enough to know what a proper fix might be, but from what I can tell it appears that the second metrics request enters the Collect function and resets the value of e.up before the first one is received and processed on the other side of the channel.

[Izbitzer]

I made a fix which I believe should work. Though as I mentioned, I am far from a Go expert, so could be that it's not an optimal solution.