It was getting rather complicated to keep track of which Segment keys
were set where, and under which name.
The build script now injects a key even in test environments, but it is
unused if `IN_TEST` is truthy. This should be functionally equivalent
to the old logic. I find this simpler mainly for two reasons: there is
one less intermediate variable to keep track of now, and the `IN_TEST`
check is now directly in the module where we're constructing the
`segment` instance, rather than being referenced at a distance in a
comment.
The old setup made it difficult to turn on metrics for specific e2e
tests as well, which will be done in a subsequent PR.
Mark Stacey
ba6509c7a3