Event support in dogstatsd

[LotharSee]

This PR allow DogStatsD to send events.

The event syntax is like this:
_e{title_length,body_length}:title|body|p:priority|k:aggregation_key|t:alert_type|s:source_type_name|d:date_happened|h:hostname|#tag1,tag2
Some details:

• Only title and body are required, others are optional, order doesn't matter
• title and body length are computed based on the utf-8 length. (it allows to use "|" in the title and the body).
• New lines are replaced by two characters "\n". Which mean a "\n" in the body will be rendered as a new line.

Exemple:
_e{8,21}My title:The world is burning!|h:my_computer|#danger,on_fire

I need some review about this protocol modification before adding it everywhere (documentation, librairies, dogstatsd client in the agent, ...).

One last thing: for now, events are sent one by one. I have two solutions to fix it:

• Update our API to allow multiple events
• Use the same endpoint as the agent
  What's the best?

[alq666]

@LotharSee works as-is with python 2.4?

[LotharSee]

Not tested yet, I will.
(It should be fine, I am not using Python 2.4+ functions).

[conorbranagan]

Maybe a dumb question, but is there a max size of these packets? Can you write an event that's too big to fit into one packet?

[LotharSee]

Yeah the maximum size is 8K.

I will add a limit in the dogstatsd client.
For the server, there is nothing specific that we can do. The truncated content may create a truncated event or be skipped if it doesn't respect the protocol.

[elijahandrews]

@LotharSee What's the status of this? Should it go in 3.8.0 (the release I'm building this week)?

[LotharSee]

I prefer to keep it away form this release.
This PR is here to facilitate reviews, I won't merge it until it is really useful (I guess after the 3.8.0).

[clofresh]

Is there a way to avoid hard coding these keys and just pass along keys to the server which would do the validation? Having these on the client side makes it difficult to change them.