Break off main function to make module importable

[n-oden]

Also update reqs to go 1.17 and apply gofmt.

This should resolve #4

[n-oden]

Hey @pengbo0328 any thoughts on this and #11 ?

[pengbo0328]

Thank you!

Could you update "Building and running" section of README.md to describe how to create and run the binary?

[n-oden]

@pengbo0328 I've taken another pass at this. The instructions in the README should still be correct: make should create the binary in the top of the repo.

[pengbo0328]

@n-oden Thank you! It works and the the binary is created in the top of the repo now.
Do you think we should rename file cmd/pgpool2_exporter/pgpool2_exporter.go to other name?

[n-oden]

@pengbo0328 good idea -- all done now!

[pengbo0328]

@n-oden Thank you!