Make the Discovery throtling configurable to allow easier testing.

[krasi-georgiev]

prometheus/discovery/manager.go

Line 134 in 75bd348

ticker := time.NewTicker(5 * time.Second)

Sending the discovery updates are hard coded to throttle at 5 seconds.
This makes creaating test for these parts of the code needing to wait 5 seconds so not good for unit testing. We need to make this somehow configurable so that we can adjust when creating unit tests.

With this change to the same PR can also add a test for:

• blocked or no updates when the receiver's channel is full - #4523 (comment)
• send updates only when there was an actual change in the targets - #4545

[janickic]

I'm going to try to fix this.