Updating guides on generating definitions

[nickcharlton]

I've been playing around with using Prometheus' remote_write in Ruby, I was finding that using Google's protobuf library was doing incorrect things, but this ended up working out great.

That said, I did find that the Compiling Definitions wiki page was leading me astray a little and I'd like to update it.

Whilst the Rake example does work fine, the example of compiling yourself directly using protoc doesn't use the correct Ruby plugin. I figured out that this works well:

$ ls definitions
prometheus.proto
$ protoc --plugin=protoc-gen-ruby-protobuf=$(which protoc-gen-ruby) --ruby-protobuf_out=lib -I definitions definitions/prometheus.pb.rb

I'd like to update the Wiki page, but before I do, could someone confirm that's broadly the right way to go about things?

Along the way I ran into #345, #341 and #382 so potentially referencing those might help others trying to do the same.