Follow go mod versioning rules and update the module name

[vihangm]

The reason this package doesn't include releases after v2.5.0+incompatible on pkg.go.dev is that the go.mod file was added between v2.5.0 and v.2.6.0 but the module name wasn't compatible with go module semver rules.

This updates the module name. This ensures that go get github.com/prometheus/prometheus will fetch v1.8.2 but
go get github.com/prometheus/prometheus/v2 will fetch v2.33.4

See https://research.swtch.com/vgo-import and https://go.dev/blog/v2-go-modules for more info.

[tomwilkie]

Theres an active discussion on this topic going on on #8852 and I don't think we're reached a consensus yet on what we'd like to do. I suggest you chip in there, and lets get this topic added to the dev summit next week to discuss.

[vihangm]

Whoops, sorry for the noise. I tried searching through the issues but didn't find that one. Thanks for the pointer. Closing this and will wait for resolution on the current discussion.