Avoid sheet name clashes on export, allow overriding of sheet name

Metrics can have the same name (or same first 31 characters). It causes a "duplicate worksheet name" exception to be thrown on export. This is solved by appending a counter at the end of the sheet name.

Metric names over 31 characters are truncated on export. Added an `export_name` field so that a user-friendly sheet name can be specified.