Fault Tolerance 1.1 spec is missing property to disable metrics integration

[Azquelt]

When we added metrics integration in #257, we specified a property to disable the metrics integration: MP_Fault_Tolerance_Metrics_Enabled

When we updated the spec to allow annotations to be explicitly enabled in #275, we accidentally removed the section that referred to MP_Fault_Tolerance_Metrics_Enabled

The TCK that tests that this property works is still present: https://github.com/eclipse/microprofile-fault-tolerance/blob/master/tck/src/main/java/org/eclipse/microprofile/fault/tolerance/tck/metrics/MetricsDisabledTest.java