Add a simple buildInfoCollector

[beorn7]

This is now kind of meant as complementary to https://github.com/povilasv/prommod , see doc comment. https://github.com/povilasv/prommod is concerned with the dependencies, while this simple collector is only doing the main module. It is, if you want, the poor person's version of prometheus/common/version. If you build your binaries in the "right" (modules) way, you get the version info for free without setting any build variables.

Since many users might not build their binaries in that "right" way yet, and some might not want to disclose their package path or version, this collector is not registered with the default registry.

[beorn7]

Yeah, the example needs to get a build tag, too.

[beorn7]

@povilasv What do you think?
@brian-brazil Does this fix #539 in the intended way?

[beorn7]

This works, but I'll change the file structure to not repeat so much doc comment.

[beorn7]

OK, all done.

[brian-brazil]

👍

[beorn7]

You can try it out:

$ GO111MODULE=on go get github.com/prometheus/client_golang/examples/random@8576729
go: finding github.com/prometheus/client_golang/examples/random 8576729
go: finding github.com/prometheus/client_golang/examples 8576729
go: finding github.com/prometheus/client_golang 8576729
$ random &
[1] 18036
$ curl -s localhost:8080/metrics | grep go_build_info
# HELP go_build_info Build information about the main Go module.
# TYPE go_build_info gauge
go_build_info{checksum="h1:4ngA1sNiO1owrwGu/W+yaCOpGrIVnhf7MYTxQMXwtFM=",path="github.com/prometheus/client_golang",version="v0.9.4-0.20190606202423-8576729b6f65"} 1

[beorn7]

Merging now. @povilasv your input is still welcome.

[povilasv]

Nice work :) Read thru the code and tested examples, everything works as documented.