CDRIVER-6080 fix stream tracking in /Client/exhaust_cursor/pool

[kevinAlbs]

Follow-up to #2095 intended to fix observed test failures on macOS replica sets. Example:

test error in: /Users/ec2-user/data/mci/c4f6b76328d456c45e1a0f91c797f34d/mongoc/src/libmongoc/tests/test-mongoc-exhaust.c 202:test_exhaust_cursor()
Got unexpected total stream count to localhost:27018:
    Expected 3, got 4

I reproduced the error by running test in repeat 100x: https://spruce.mongodb.com/version/68bb4bca9fc72c00079c4923
Tests pass with this fix applied: https://spruce.mongodb.com/version/68bb4f4fe5543500071df4fc

I expect the extra observed connection is due to the monitoring connections taking time to establish. This fix awaits monitoring connections before proceeding with the test.

[rcsanchez97]

LGTM