Hello,
Thanks for reaching out. The GenerateHistoricalMetricsRequest method can only accept one keyword plan, as you mentioned. This method also is not supported for batch jobs, as the BatchJobService only supports mutate operations (you can see a list of supported batch job operations
here).
The planning services and KeywordPlanService metric generation methods are subject to special limits. You can find these
here.
Regards,
Matt
Google Ads API Team

ref:_00D1U1174p._5004Q2QEElq:ref