Model validation errors for metricAlerts.json examples

Found the CIs ([here](https://travis-ci.org/Azure/azure-rest-api-specs/jobs/395287731))reporting a bunch of model validation errors from an unrelated PR [here](https://github.com/Azure/azure-rest-api-specs/pull/3285#pullrequestreview-131695350)
Examples/APIs must be fixed accordingly

Start with the linked Travis CI job and the metricAlerts.json examples to identify each reported model validation error. Compare the affected examples with the API specification expectations, then update the examples/APIs until the validation checks pass without introducing unrelated changes.

Written by the indexing model from the issue text.