Add -redis.file option back to support monitoring of multiple Redis dbs

[roman-vynar]

Fixes #278

This makes redis_exporter great again.

The changes made are fully backward-compatible with the current functionality. I mean it will still behave the same when you pass -redis.addr or try to access exporter using /scrape?target=<dynamic host>.

Again:

1. It does neither harm nor change the current behaviour/performance.
2. It allows you to monitor multiple instances w/o running multiple exporters.
3. A single password for multiple instances or no password, passing passwords from prometheus are all insecure approaches.
4. Maintaining prometheus discovery + redis_exporter mapping host<>password doubles the operational overhead, thus -redis.file is an old good option.

Thanks!

[oliver006]

Thanks for the PR but I'm not going to bring back the functionality to monitor multiple redis instances by providing config options to the exporter (e.g. a file or cmd line params).

There are other ways to do this, among them the solution sketched out by Brian in the PR here which IMO is preferable.

[roman-vynar]

No problem.

As mentioned above, the solution suggested introduces more operational overhead and does not solve the password problem.