public static HttpHeaders mergeTrailers(HttpHeaders headers, HttpHeaders additionalTrailers) {
        if (additionalTrailers.isEmpty()) {
            return headers;
        }
        if (headers.isEmpty()) {
            return additionalTrailers;
        }

        final HttpHeadersBuilder builder = headers.toBuilder();
        for (AsciiString name : additionalTrailers.names()) {
            //  Pseudo-headers are disallowed for additional trailers
            if (!isPseudoHeader(name) &&
                !isTrailerDisallowed(name)) {
                builder.remove(name);
                additionalTrailers.forEachValue(name, value -> builder.add(name, value));
            }
        }
        return builder.build();
    }
Merges the given {@link RequestHeaders}. The headers have priority in the following order.
<pre>{@code
additional headers (highest priority) > headers > default headers > framework headers (lowest priority)
}</pre>
core/src/main/java/com/linecorp/armeria/internal/common/HttpHeadersUtil.java