Support Apache OpenWhisk lambda functions

[rabbah]

With a few small localized changes, this library will work with Apache OpenWhisk - an open source platform for serverless functions powering public clouds (IBM Cloud Functions, Adobe I/O, Nimbella) and many others offerings on-prem.

There are two main differences between OpenWhisk and AWS:

1. the "main" handler signature is main(event)in OpenWhisk vs main(event, context) in Lambda. The context is provided in environment variables in the former.

2. the method utilts.get_fields is specific to AWS.
   

   lambda-python/signalfx_lambda/utils.py

   Line 7 in a6bf942

   def get_fields(context):

To keep changes minimal, I created a method to convert an OpenWhisk environment to an AWS-like context object (so get_fields remains agnostic to this change). I've confirmed the change is sufficient to allow OpenWhisk functions running on Nimbella to use the Lambda dashboards on SignalFx.

I can submit this patch as a pull request if desirable (do you have a contribution guide?), along with some documentation to show an OpenWhisk example as in this repo https://github.com/rabbah/sfx.

Here's the OpenWhisk lambda using the SignalFx decorator.

https://github.com/rabbah/sfx/blob/11c4264661a2d6c80f51e7f41ff0938174ac1e83/packages/default/hello.py#L1-L8

[slernersplunk]

FYI for Splunk SignalFx GitHub approvers... - I (SignalFx Splunk Senior Sales Engineer) am working with Rodric and Nimbella on this and am creating OpenWhisk and Nimbella demos...