opencensus: tweak enum in configuration API. (#7280)
opencensus: tweak enum in configuration API. (#7280)

Since this API is still experimental, tweaking to match best proto
practices.

Risk level: Low

Signed-off-by: Harvey Tuch <htuch@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ 1faaed85740a97533484db3232796aef7973677f

lyft/envoy-api,lyft/envoy-api,lyft/envoy-api