Add list of available metrics to README

[jsha]

For someone evaluating whether to give this exporter a try, it's very useful to know which metrics it makes available.

[agirot]

@jsha Can you fix all checks plz ? Your PR is useful :)

[jsha]

I added the DCO but now I'm getting a seemingly unrelated Travis failure:

package golang.org/x/tools/go/loader: unrecognized import path "golang.org/x/tools/go/loader" (parse https://golang.org/x/tools/go/loader?go-get=1: no go-import meta tags ())

[simonpasquier]

@jsha it looks like a transient error on the Travis side. I've restarted it and it's all green now.

[jsha]

I generated the list by running the haproxy_exporter and curling the metrics port, then running it through a handful of string replacements in Vim. I could systematize that process and document it here, but it might take a little while.

I agree the http_ prefixed stats are confusing in this context and should go. Should this list contain only the haproxy_ prefixed stats?

[matthiasr]

Unfortunately, at least as of #236 this list is out of date again. The recommended way to get metrics from HAProxy is the official exporter, which has a documented list in this file. Since this PR is >5 years old, I'll close it.