Extend receiverhelper Scraping library

[james-bebbington]

To resolve #934, we created a library to simplify creating receiver components that "scrape" data, see scraper & scraper controller in: https://github.com/open-telemetry/opentelemetry-collector/tree/master/receiver/receiverhelper

After applying this library to the hostmetrics receiver, my thoughts were that we could make this library even more usable if we:

• Add ScraperFactory interfaces
• Add code to simplify creation of scrapers via factories to receiver/receiverhelper

At that point, the Scraper interfaces will have many similarities in structure with other components, and it may arguably make sense to:

• Move interfaces from receiver/receiverhelper/scraper.go into component/scraper.go

I don't think that scrapers should be a "top level component" though in the sense that they should be configurable at the top level like receivers / processors / exporters. Here's a doc @jrcamp and I were using a while back to discuss the idea of making Scraper a top level component which eventually led us to do none of the things in that doc and implement this as a library in receiverhelper instead: https://docs.google.com/document/d/15WKpYvw23rQ6WnBhoE2wdk4szfjlXEXg8WYYvBcKshA/edit