common/tracer: fix decoding when jaeger tracing is disabled

We aren't currently using jaeger tracing on Windows. The issue is
that Windows hosts (or any other host that doesn't use jaeger)
are experiencing message decoding failures after a recent change [1].

This change updates the tracer encoding so that messages from
non-jaeger hosts may be decoded by services that use jaeger.

[1] ceph#47457

Signed-off-by: Lucian Petrut <[email protected]>

This commit rebrings 3701ffa which
got reverted due to an implicit dependency with other revert. Please
see ceph#52114 (comment).

Conflicts:
	src/common/tracer.h
	  formatting conflict with 7179ac0

Author: petrutlucian94

src/common/tracer.h

@@ -152,9 +152,20 @@ struct Tracer {
   jspan_ptr add_span(std::string_view span_name, const jspan_context& parent_ctx) { return {}; }
 };
 
-inline void encode(const jspan_context& span, bufferlist& bl, uint64_t f=0) {}
-inline void decode(jspan_context& span_ctx, ceph::buffer::list::const_iterator& bl) {}
+inline void encode(const jspan_context& span_ctx, bufferlist& bl, uint64_t f = 0) {
+  ENCODE_START(1, 1, bl);
+  // jaeger is missing, set "is_valid" to false.
+  bool is_valid = false;
+  encode(is_valid, bl);
+  ENCODE_FINISH(bl);
+}
 
+inline void decode(jspan_context& span_ctx, bufferlist::const_iterator& bl) {
+  DECODE_START(254, bl);
+  // jaeger is missing, consume the buffer but do not decode it.
+  DECODE_FINISH(bl);
 }
 
+} // namespace tracing
+
 #endif // !HAVE_JAEGER