Implement a UDP client

[sparrc]

closes #4647

[otoolep]

Looks good to me.

Should NewClient now be renamed to NewHTTPClient? If this v2 client code is not yet frozen, I think you should consider that. It feels more Go-like.

[sparrc]

@corylanou @jwilder @otoolep My 1st precision implementation wasn't working right, so I've added a function to models.Point, RoundedString(d time.Duration) string. Main use case here is so that we can write UDP points with precision.

[sparrc]

@otoolep That's a good question, maybe I could have NewClient log a warning DEPRECATED message, and call NewHTTPClient. What do you think?

[otoolep]

@sparrc -- have we made any promises re V2 yet?

[sparrc]

@otoolep I'm not sure what you mean by promises, but we do have this note telling people to use it already: https://github.com/influxdb/influxdb/tree/master/client#description

[otoolep]

OK, that is what I mean.

OK, I would scrap the idea of renaming and leave NewClient as is, and add NewUDPClient like you are doing. A HTTP client is what most people should be using anyway.

[sparrc]

@otoolep I made some significant changes based on what we were talking about in Slack. Now the UDP client will write a UDP packet before we hit the maximum size (64kb) and then reset the buffer

[otoolep]

While this will probably work in practise, this is not watertight. Instead the logic should be checking that if adding the new point to the existing buffer will send the buffer over the limit, then call con.Write() before adding the new point.

Make sense?

[otoolep]

Unless you can be sure that a point coming in will not be greater than ~1K, but still, this logic shouldn't care if it's watertight.

[sparrc]

It's a better idea, I was worried about the extra processing, but I'll benchmark it and see

[sparrc]

@otoolep I've pushed the changes you suggested about the buffer size before writing, and I also made changes to make the godoc examples look better, as well as an update to the README

[otoolep]

Don't forget the CHANGELOG.

[otoolep]

I presume it's OK to write b if len(b) is 0.

[otoolep]

I think this looks fine. I do think you need to actually test that packets sent using this code actually works -- code a throwaway client that uses UDP and sends data to an InfluxDB service, and check that the data is in InfluxDB. Since it's UDP, this unit testing doesn't guarantee it is working. Just to be sure.

Once you do that, +1