feat(telemetry): report malformed baggage header

src/datadog/telemetry_metrics.cpp

@@ -56,6 +56,8 @@ const telemetry::Counter extracted = {"context_header_style.extracted",
                                       "tracers", true};
 const telemetry::Counter truncated = {"context_header.truncated", "tracers",
                                       true};
+const telemetry::Counter malformed = {"context_header_style.malformed", "tracers",
+                                      true};
 }  // namespace trace_context
 
 }  // namespace tracer

src/datadog/telemetry_metrics.h

@@ -109,6 +109,10 @@ extern const telemetry::Counter extracted;
 /// reason for truncation (`truncation_reason:baggage_item_count_exceeded`,
 /// `truncation_reason:baggage_byte_count_exceeded`)
 extern const telemetry::Counter truncated;
+
+/// The number of times baggage headers are dropped because they're malformed
+/// (missing key/value/'='), tagged by header style (`header_style:baggage`)
+extern const telemetry::Counter malformed;
 }  // namespace trace_context
 
 }  // namespace tracer

src/datadog/tracer.cpp

@@ -449,6 +449,11 @@ Expected<Baggage, Baggage::Error> Tracer::extract_baggage(
   if (maybe_baggage) {
     telemetry::counter::increment(metrics::tracer::trace_context::extracted,
                                   {"header_style:baggage"});
+  } else if (auto err = maybe_baggage.if_error()) {
+    if (err->code == Baggage::Error::MALFORMED_BAGGAGE_HEADER) {
+      telemetry::counter::increment(metrics::tracer::trace_context::malformed,
+                                    {"header_style:baggage"});
+    }
   }
   return maybe_baggage;
 }