Provide metadata to intake shaper

[alfred-landrum]

When a shaper is executed on received data, we can supply potentially useful metadata to the shaper code by making it available as a pre-defined value with a well known name. The metadata could include information about the intake, like its id & its target space. It could also provide any out-of-band information from the received data or transport; for example, in the case of the elasticsearch bulk api, this could be a means to provide the index name used in the bulk api request.

[alfred-landrum]

#2242 is an implementation of a Z function parse_zson() that could be used for providing the metadata to shapers.