Handling shutdown signal gracefully; minor updates

[logyball]

Closes #19

Adding a graceful shutdown to the exporter, complete with a timeout for the server. Additionally, added some minor refactors throughout the exporter.go file to make the code slightly more readable.

Shutdown Changes

• Added a context that listens for SIGINT, SIGTERM
• Moved the server into its own http.Server{} struct
• block on context finishing

Refactors

• moved additional config flags specific to providers into ProviderFlags()
• moved other flags into operationalFlags()
• Added a server timeout configuration item - added to the config object
• Removed line about default GCS collection from README (we don't collect anything by default)
• Moved some functionality to init() and createPromRegistryHandler() helper functions

[logyball]

I'm gonna pass on the os.exit() suggestion for now - this seems like a nice-to-have that we won't need and adds a little bit of clutter. I think it'd be surprising if we used different exit codes for something meaningful, and we can re-evaluate when changing the log -> slog if we feel strongly.