We now have two write interceptors, with one moved into
faas-provider. This commit makes the gateway use the new
external package and deletes its own.

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alex@openfaas.com>

* This experimental patch records metrics as invocations start
so that the metrics can be used to make better scale to zero
decisions in faas-idler.

Tested with Kubernetes on a single-node cluster, metrics
reported as expected. Existing metrics still report.

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alexellis2@gmail.com>

- fixes issue where result was assigned to value rather than
to pointer reference.

Signed-off-by: Alex Ellis (VMware) <alexellis2@gmail.com>

Signed-off-by: Alex Ellis (VMware) <alexellis2@gmail.com>