Title: Support outlier detection of other status codes (particularly 4xx).

Description:
Outliers can be hosts returning an abnormal rate of any status code, not just 5xx. Although 4xx errors are generally considered client errors, if a host starts returning a large number of 4xx, it may signal it has some problem (possibly related to authz, authn, etc) and should be considered an outlier. At Pinterest, we are interested in being able to identify 4xx outliers in addition to 5xx outliers (although I can imagine this could have a general solution for all 300+ status codes).

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