Net.HttpAuthPromptType histogram (RecordHttpAuthPromptType() in login_handler.cc) currently records four buckets:

AUTH_PROMPT_TYPE_WITH_INTERSTITIAL (Auth prompt is shown with a blank interstitial)
AUTH_PROMPT_TYPE_MAIN_FRAME (Auth prompt is shown for the main frame document)
AUTH_PROMPT_TYPE_SUBRESOURCE_SAME_ORIGIN (Auth prompt is shown for a subresource with same origin as the top document)
AUTH_PROMPT_TYPE_SUBRESOURCE_CROSS_ORIGIN (Auth prompt is shown for a subresource with sa different origin than the top document)

However these don't differentiate between server and proxy auths. We might want to buckets for proxy specific prompts (indicated by auth_info.is_proxy) to narrow down the root cause of extraneous prompts.