add generic http input plugin which supports setting an input data format

[grange74]

Issue #813.

Required for all PRs:

• Signed CLA.
• Associated README.md updated.
• Has appropriate unit tests.

[danielnelson]

This is actually optional, if unset it will default to "influx". Just use the default snippet here:

  ## Data format to consume.
  ## Each data format has its own unique set of configuration options, read
  ## more about them here:
  ## https://github.com/influxdata/telegraf/blob/master/docs/DATA_FORMATS_INPUT.md
  # data_format = "influx"

[danielnelson]

Because of my comment about data_format we should set this to something more format neutral such as http://localhost/metrics

[danielnelson]

Don't set a response header timeout since we have the client timeout. I need to exercise these from the code so they stop getting copied around.

[danielnelson]

Update the comment based on removing header timeout (see below). Use the default value and comment it out:

# timeout = "5s"

[danielnelson]

I may have mentioned that I'm hoping to deprecate the httpjson plugin. I looked through its feature set to see what we might still need. (None of these are required to have this merged)

The httpjson plugin has support for setting general headers, and also using the POST or GET method. I think these are important options we will need.

It also adds a tag server to every metric with the actual url. I think this is also important but it seems that you should be able to configure if you want this tag added, since it could conflict with data provided at the url. Perhaps we should have an option tag_url = false, and instead of calling this tag server we should call it url?

A float field is added to store the response_time. This one I don't think we should add, it doesn't fit the purpose of the plugin in my opinion. If there is strong demand for this then perhaps this could be a selfstat aka internal metric.

[grange74]

I have updated based on your 4 comments.
In the next few days, i will look to add support for setting generic headers, POST method and optional tagging with url.

[danielnelson]

I added a few things after merging, notably method support and I decided to change the url tagging to method to automatically tag with url if it is not set in the remote host. I decided to make this change because I felt like it was closer to the way the measurement name is set.

[grange74]

@danielnelson Thanks for merging the PR and explaining your changes. The changes make perfect sense.