rename opentelemetry-exporter-cloud-* to opentelemetry-exporter-gcp-*

[aabmass]

Follow up to #107. We have decided on opentelemetry-exporter-gcp-{trace,monitoring} names. Pr created by:

git mv opentelemetry-exporter-cloud-monitoring/ opentelemetry-exporter-gcp-monitoring
git mv opentelemetry-exporter-cloud-trace/ opentelemetry-exporter-gcp-trace/
perl -i -p -e 's/opentelemetry-exporter-cloud-monitoring/opentelemetry-exporter-gcp-monitoring/' `git ls-files`
perl -i -p -e 's/opentelemetry-exporter-cloud-trace/opentelemetry-exporter-gcp-trace/' `git ls-files`