Add a UUID to injected metadata

[jchesterpivotal]

The problem

Currently, the resource injects 4 metadata (its release version, its git ref, the timestamp and the remote Concourse version).

It injects these into all of the JSON files it produces. This makes it easy to get basic provenance of a record inserted into a database.

What I would like to have is a trustworthy unique identity for any particular get. None of the existing metadata, singly or in combination, is able to do this. In particular there's a temptation to assume that together they will be unique, but that's not true: I could be running jobs in parallel. Or I might be running the resource in multiple places and pooling the data.

A solution

To guarantee uniqueness, add a UUID to the injected metadata. This simplifies insert logic and lets me tie together all the information ever retrieved on a particular occasion.

[jchesterpivotal]

I also added it to the version metadata, but I am having second thoughts. This value will change every time the resource runs, but in theory, the build it refers to does not. So in fact changing the version metadata with an ever-changing UUID really creates a new version, which I don't want.

But the UUID for the rest of this stuff seems quite sound.