DRIVERS-3217 Investigate error rate tracking for server selection#2

ShaneHarvey wants to merge 2 commits intobackpressureShaneHarvey/mongo-python-driver:backpressurefrom

DRIVERS-3217-poc-error-rate-server-selectionShaneHarvey/mongo-python-driver:DRIVERS-3217-poc-error-rate-server-selectionCopy head branch name to clipboard