[discovery-gce] Align code examples and documentation (#28876)
[discovery-gce] Align code examples and documentation (#28876)

The docs state that `_gce_` is recommended but the code sample states
that `_gce:hostname_` is recommended. This aligns the code sample with
the documentation. Also replace `type` with `zen.hosts_provider` as 
discovery.type was removed in #25080.

vroyer/elassandra,vroyer/elassandra,strapdata/elassandra,vroyer/elassandra,strapdata/elassandra,strapdata/elassandra,strapdata/elassandra,strapdata/elassandra