Fix fallback metric

Naming it to `MemcachedFallbackMetric`, because this is counting from the `MemcachedFallback`. Even though the content api fallbacks are the only thing using it.

This is still being pushed into `CloudWatch` as `content-api-fallbacks`. Only doing this to see the effect on the current graph, but maybe should generalize it there too?

@robertberry @stephanfowler