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

envoy/config/trace/v2/trace.proto

@@ -134,14 +134,17 @@ message OpenCensusConfig {
   string zipkin_service_name = 7;
 
   enum TraceContext {
+    // No-op default, no trace context is utilized.
+    NONE = 0;
+
     // W3C Trace-Context format "traceparent:" header.
-    trace_context = 0;
+    TRACE_CONTEXT = 1;
 
     // Binary "grpc-trace-bin:" header.
-    grpc_trace_bin = 1;
+    GRPC_TRACE_BIN = 2;
 
     // "X-Cloud-Trace-Context:" header.
-    cloud_trace_context = 2;
+    CLOUD_TRACE_CONTEXT = 3;
   }
 
   // List of incoming trace context headers we will accept. First one found