Bug: --monitoring.filters flag broken when filter value contains : character

Root cause:
In stackdriver_exporter.go the SplitExtraFilter function uses : as a separator to split the metric prefix from the filter value:

`utils.SplitExtraFilter(ef, ":")

This breaks when the `database_id` value contains `:`, for example:

project:instance`

Fix:

`sed -i 's/utils.SplitExtraFilter(ef, ":")/utils.SplitExtraFilter(ef, "|")/' stackdriver_exporter.go

This changes the separator to `|`, so flags are written as:

--monitoring.filters=cloudsql.googleapis.com/database/up|resource.labels.database_id="project:instance"`

Steps to reproduce:

1. Set --monitoring.filters=cloudsql.googleapis.com/database/up:resource.labels.database_id="project:instance"
2. Exporter does not apply the filter and returns metrics for all instances

Expected: Filter is applied, only the selected instance is returned
Actual: Filter is silently ignored, all instances are returned
Environment:

• stackdriver_exporter master branch (go 1.24.0)
• Google CloudSQL database metrics
• database_id format: project-id:instance-name (contains :)

Start in stackdriver_exporter.go at the --monitoring.filters handling and its SplitExtraFilter call. Reproduce the CloudSQL case with a database_id containing a colon, then verify that the filter is applied and only the selected instance's metrics are returned.