Fix bug corrupting Prometheus timeseries __name__ label

[alibresco]

I have also been experiencing issues like the ones described in #259 and #169 where Prometheus outputs are sometimes dropped or incorrect.

I believe this is due to a bug with how the __name__ label gets added to the list of labels when constructing a timeseries. The __name__ label is appended without a copy or assignment, meaning it will overwrite whatever was appended previously, so long as there is space remaining in the array. If GetLabels returns a slice with length == capacity, the bug does not seem to occur because append forces a copy.

A consequence of this bug is that a value can get written to the wrong timeseries.

I've written a test that reproduces this behavior. The test fails without the fix and passes with the fix.

I don't write a lot of Go, so let me know if I've gotten anything wrong here or there's a better way to solve this. I stumbled upon this issue while investigating incorrect gNMIC Prometheus writes that I was observing.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[spolack]

Can you please provide a amd64 build? I can confirm whether this fixes #259

[karimra]

@alibresco This makes sense, I will try it out and merge if all good.

[karimra]

Thanks for the contribution and for writing tests.