Add two new metrics to monitor the client-go logic that
generate http.Transports for the clients.
- rest_client_transport_cache_entries is a gauge metrics
with the number of existin entries in the internal cache
- rest_client_transport_create_calls_total is a counter
that increments each time a new transport is created, storing
the result of the operation needed to generate it: hit, miss
or uncacheable
Change-Id: I2d8bde25281153d8f8e8faa249385edde3c1cb39
