Add String() implementation for MetricInfo
Add String() implementation for MetricInfo

This commit adds an implementation of `String()` for
provider.MetricInfo, which should make printing errors and debug
messages easier.

GoogleCloudPlatform/k8s-stackdriver,erocchi/k8s-stackdriver,GoogleCloudPlatform/k8s-stackdriver,erocchi/k8s-stackdriver,erocchi/k8s-stackdriver,kawych/k8s-stackdriver,kawych/k8s-stackdriver,erocchi/k8s-stackdriver,kawych/k8s-stackdriver,kawych/k8s-stackdriver,GoogleCloudPlatform/k8s-stackdriver