Feature/observers

[fralalonde]

No description provided.

[fralalonde]

• Still missing self.notify_flush_listeners() for most output's flush()
• on_flush requires metrics to be mut
• the behavior for adding multiple flush listeners on sibling outputs needs to be sorted out

[fralalonde]

I'm not sure about the Observer struct being worth much... It could be easily replaced by a Gauge-updating closure wherever it is used, either on_flush or every scheduled.

[fralalonde]

@mixalturek last call for API changes, this is ready to merge!

I'm satisfied with the new API and I believe I went back to something close to your original proposal:

    let uptime = metrics.gauge("uptime");
    metrics.observe(uptime, || 6).on_flush();

    metrics
        .observe(metrics.gauge("threads"), thread_count)
        .every(Duration::from_secs(1));

What do you think?

I'm sorry for taking so long and clobbering your original PR... Some lower-level changes were required for this to work, mainly the flush notifications and a new shared-thread scheduler (so we don't create a new thread for every observed gauge).

Your original commits are still in the commit log for posterity, if it's something you care about :)

[mixalturek]

Hi, I'm sorry, I had and have too many other tasks from many directions and didn't have time for this at all. I also received no email notification about your work until yesterday, so I fully missed everything. The API described in the comment above looks great!