traceparentからコンテキストを抜いてリモートでParentを指定する. ref: https://github.com/census-instrumentation/opencensus-java/blob/master/impl_core/src/main/java/io/opencensus/implcore/trace/propagation/TraceContextFormat.java

OpenCensusContextSample.java

String traceparent = headers.getRequestHeaders().getFirst("traceparent");
TraceId traceId = TraceId.fromLowerBase16(traceparent, TRACE_ID_OFFSET);
SpanId spanId = SpanId.fromLowerBase16(traceparent, SPAN_ID_OFFSET);
TraceOptions traceOptions = TraceOptions.fromLowerBase16(traceparent, TRACE_OPTION_OFFSET);

try (Scope ss = tracer
        .spanBuilderWithRemoteParent("Span Name", SpanContext.create(traceId, spanId, traceOptions))
        .setRecordEvents(true)
        .setSampler(Samplers.alwaysSample())
        .startScopedSpan()) {

    // do somthing.
}