Research possibilty to allow metadata to be set through the request env

[tombruijn]

Add the metadata in the integration middleware and read that from the generic rack middleware as described in #329

request.env["Appsignal.params"] = { "foo": "bar" }
# request.env["appsignal"]["params"] = { "foo": "bar" }
# request.env["appsignal"].params = { "foo": "bar" }

and then we'll capture that in the rack middleware ensure and set it on the transaction there.

[tombruijn]

Not sure if we should still do this. We have helpers for it.
If no one else disagrees close then the next time I see it.