cloud-api: unbounded HTTP method label in request metrics #172

Found by the Security review of nan-cloud-api#337. The method label of the cloud-api HTTP request metrics takes arbitrary HTTP methods from clients, so anyone can create unbounded series (cardinality). Fix: map anything outside the standard methods to OTHER, with a test. Follow-up from QA: pin the /nan_whk_AAAAAAAAnan_whk_BBBBBBBB path case in the logger redaction tests.