Support fo go modules

[badrabubker]

Hi,
thanks for providing elasticsearch-exporter. I was trying to package this exporter for debian.

First issue i had is that prometheus-client-golang in the vendor directory is outdated and newer versions break computability with the < 1 versions .
So i patched that in the package.
I tired to vendor it as well, however the newer prometheus-client-golang depends on github.com/cespare/xxhash/v2 which needs go modules

I would be happy to submit a pull request if you would like :)

Thanks !
Cheers,
Badr