cmd/fastly-exporter: move roundTripperFunc, userAgentTransport into main.go, allowing "go run cmd/fastly-exporter/main.go"

[shawnps]

No description provided.

[peterbourgon]

go run is meant as a convenience for trivial, generally single-file, script-ish Go programs. It's not really something that should be used with projects like this. It calculates the source that goes into the compilation unit in a way that's different than all other go tool subcommands, which is confusing -- hence this PR! 😉 And it doesn't take advantage of intermediate build caching to the extent that go build and/or go install do.

Anyone working on this project should be building-and-running via

go build ./cmd/fastly-exporter 
./fastly-exporter ...

or

go install ./cmd/fastly-exporter
fastly-exporter ...

But if you insist on using go run 😉 then you should be able to go run cmd/fastly-exporter — i.e. eliding the main.go — which should work without requiring the change in this PR?