Explicitly forward metricVec methods Collect, Describe, Reset
Explicitly forward metricVec methods Collect, Describe, Reset

Interestingly, methods implicitly forwarded from embedded types are
detected by GoDoc if they are just one level deep. Embedded types in
the embedded type are not recognized. This commit therefore adds
explicit forwarding methods for Collect, Describe, and Reset.

Signed-off-by: beorn7 <91e32ae1d26bd312b370cf8378a5a83accca90a0@grafana.com>

m3db/prometheus_client_golang,prometheus/client_golang