Fix metrics filter with colon : in query value

[dn0]

Example of a metric filter affected by this:

cloudsql.googleapis.com/database:resource.labels.database_id=project-id:database-name

[dn0]

@SuperQ @kgeckhart Feel free to close this one in favor of anything that will fix this :)

The problem is visible in debug logs

level=INFO source=stackdriver_exporter.go:366 msg="Starting stackdriver_exporter" version="(version=0.18.0, branch=HEAD, revision=3b1a6e9ebb85a176b8ab48639f8e0a2a4a2f7bcf)" build_context="...." metric_prefixes="[... cloudsql.googleapis.com/database/disk/bytes_used cloudsql.googleapis.com/database/disk/quota cloudsql.googleapis.com/database/up]" extra_filters="cloudsql.googleapis.com/database:resource.labels.database_id=my-project-id:my-database" projectIDs=[my-project-id] projectsFilter=""

level=DEBUG source=monitoring_collector.go:285 msg="retrieving Google Stackdriver Monitoring metrics for descriptor" project_id=my-project-id descriptor=cloudsql.googleapis.com/database/disk/bytes_used

level=DEBUG source=monitoring_collector.go:315 msg="retrieving Google Stackdriver Monitoring metrics with filter" project_id=my-project-id filter="metric.type=\"cloudsql.googleapis.com/database/disk/bytes_used\" AND (resource.labels.database_id=my-project-idmy-database)"