Next major version: Changes to consider

[knweiss]

This issue is a wip collection of (compatibility-breaking) changes to consider for the next major version:

• The package name currently is grpc_prometheus. However, the recommendation for Go package names is "Good package names are short and clear. They are lower case, with no under_scores or mixedCaps." Consequently golint complains:

client.go:6:1: don't use an underscore in package name
client_metrics.go:1:1: don't use an underscore in package name
client_reporter.go:4:1: don't use an underscore in package name
client_test.go:4:1: don't use an underscore in package name
metric_options.go:1:1: don't use an underscore in package name
server.go:6:1: don't use an underscore in package name
server_metrics.go:1:1: don't use an underscore in package name
server_reporter.go:4:1: don't use an underscore in package name
server_test.go:4:1: don't use an underscore in package name
util.go:4:1: don't use an underscore in package name

• Consider unexporting the four constants in util.go (golint):
  util.go:16:2: exported const Unary should have comment (or a comment on this block) or be unexported

[bwplotka]

Thanks for this list, definitely package names might be something we should improve, but breaking compatibility is not great (: