tracing: Enable zipkin shared span context to be configurable, with default true (#4426)

Adds configuration option to zipkin tracer config to determine whether shared span context should be used.

Resolves #4397

Risk Level: Medium

Testing:
Currently defined unit tests but will try on some examples early next week.

Docs Changes:
Config option defined in proto.

Signed-off-by: Gary Brown <gary@brownuk.com>

Mirrored from https://github.com/envoyproxy/envoy @ 2ddebc626920f05697c8f2582a683b03c0b10991

envoy/config/trace/v2/trace.proto

@@ -10,6 +10,8 @@ import "envoy/api/v2/core/grpc_service.proto";
 
 import "google/protobuf/struct.proto";
 
+import "google/protobuf/wrappers.proto";
+
 import "validate/validate.proto";
 
 // The tracing configuration specifies global
@@ -59,6 +61,10 @@ message ZipkinConfig {
   // Determines whether a 128bit trace id will be used when creating a new
   // trace instance. The default value is false, which will result in a 64 bit trace id being used.
   bool trace_id_128bit = 3;
+
+  // Determines whether client and server spans will shared the same span id.
+  // The default value is true.
+  google.protobuf.BoolValue shared_span_context = 4;
 }
 
 // DynamicOtConfig is used to dynamically load a tracer from a shared library