More than the number of documents in a collection, what you want to be careful of is the TTL for the collection. Setting it for too long a period can result in long read times from the associated indexes, because the indexes need to account for all of that history. See the section on collections in the docs for more details.
Also, keep in mind that pagination maxes out at 100,000 documents per page. So if you have millions of documents in the collection, you’ll need to iterate through 10 or more pages to retrieve them all. Depending on the metrics being tracked there may be more efficient ways of doing that.
Can you share more details about your use case? What kind(s) of metrics are you tracking? How much history for those metrics is required?