Add prometheus metrics

[shaikatz]

Change prometheus package to use latest master (locked by revision as advised in repo) to take advantage of promauto.

Added basic app instrumentation metrics:

1. Run counter
2. Error counter
3. PodsDeleted counter
4. Pod Deletion timers

That's how it looks:

My first PR in Golang, so I'm happy to get some comments and improvements.

I can also implement that with some channels, and it'll be a good start to plugin a slack notifications.

Thoughts?

[shaikatz]

After some rethinking, changed it to channels.

Still has to fix and add tests.

[linki]

Thanks @shaikatz.

Looks fine from a Go perspective. Can we leave out the channel implementation for now?

[shaikatz]

@linki No problem, rebased to the last commit before channels.
I don't see a need for a tests with this approach.
Can we merge that?

[linki]

@shaikatz I'll have a look this week.

[zbindenren]

@shaikatz a good practice would be to prefix the chaoskube specific metrics with chaoskube_

For example pods_deleted would become chaoskube_pods_deleted etc...

This allows to easily find chaoskube specific counters.

[shaikatz]

Sorry I being in a long vacation, I see that the project advanced a bit in this time, I'll close this PR for now, thanks.