count(count by (a) (hello_info))

First you want an aggregator with a result per value of a, and then you can count them.

From storage and processing standpoint there is no much difference in those two approaches.

But recommendations on instrumenting your application with Prometheus metrics contains the following:

Metric names should never be procedurally generated, except when writing a custom collector or exporter.

Consider following: if you plan to aggregate over all providers (for example, for total or average number of requests regardless of provider), approach with separate metrics will require queries like sum({__name__ =~ "bot_success_.+"}), and it's generally advised against of. And you'd like to aggregate over provider and some other label, you'll need to extract provider from the metrics name first, and only then proceed with aggregation.

Additionally, I would recommend you to checkout best practices in the metric naming. For example, your metric most likely will end up being a counter, thus its name should be something like bot_api_requests_total.