Mint.HTTP2.open_request_count

You're seeing just the function open_request_count, go back to Mint.HTTP2 module for more information.
Link to this function

open_request_count(conn)

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Specs

open_request_count(t()) :: non_neg_integer()

See Mint.HTTP.open_request_count/1.

In HTTP/2, the number of open requests is the number of requests opened by the client that have not yet received a :done response. It's important to note that only requests opened by the client (with request/5) count towards the number of open requests, as requests opened from the server with server pushes (see the "Server push" section in the module documentation) are not considered open requests. We do this because clients might need to know how many open requests there are because the server limits the number of concurrent requests the client can open. To know how many requests the client can open, see get_server_setting/2 with the :max_concurrent_streams setting.