[integration] Temporarily pin Prometheus to prom/prometheus:v2.31.2 (#4008)
[integration] Temporarily pin Prometheus to prom/prometheus:v2.31.2 (#4008)

This works around an apparent regression in latest (v2.32.0+) when
running 'make docker-compatibility-test'.
m3db/m3db,m3db/m3db,m3db/m3db,m3db/m3db