which will be used for subsequent requests in a round-robin fashion (in
the same way we handle the actual payload / body if multiple protobuf
objects are specified).

for the metadata config option - either an object or an array of
objects.

JSON string into an object.

When this flag is used, we don't templatize metadata JSON object for
every single request and json load it, but we only do that once and
re-use cached version for subsequent requests.

This results in much less overhead when metadata template functionality
is not needed and utilizing large metadata objects.