RequestHeaders mergedRequestHeaders(RequestHeaders headers) {
        final HttpHeaders internalHeaders;
        final ClientRequestContextExtension ctxExtension = ctx.as(ClientRequestContextExtension.class);
        if (ctxExtension == null) {
            internalHeaders = HttpHeaders.of();
        } else {
            internalHeaders = ctxExtension.internalRequestHeaders();
        }
        return mergeRequestHeaders(
                headers, ctx.defaultRequestHeaders(), ctx.additionalRequestHeaders(), internalHeaders);
    }
Invoked on each write of an {@link HttpObject}.
core/src/main/java/com/linecorp/armeria/client/AbstractHttpRequestHandler.java