Expose XCMetricsBackendLib in products

[jkmathew]

• Expose XCMetricsBackendLib in products This way we can create a custom backend server with the use of additional middleware, for example, our own auth middleware. like

  
  import XCMetricsBackendLib
  import Vapor

var env = try Environment.detect() try LoggingSystem.bootstrap(from: &env) let app = Application(env) defer { app.shutdown() } app.middleware.use(AuthMiddleware()) try configure(app) try app.run()

- Fixed a couple of warnings in Package.swift by changing `.exact("x.x.x")` to `exact: "x.x.x"`