public AbstractClientOptionsBuilder setHeaders(
            Iterable<? extends Entry<? extends CharSequence, ?>> headers) {
        requireNonNull(headers, "headers");
        this.headers.setObject(headers);
        return this;
    }
Sets the default HTTP headers for an {@link HttpRequest} that will be sent by this {@link Client}.

<p>Note that the values of the default HTTP headers could be overridden if the same
{@link HttpHeaderNames} are defined in {@link HttpRequest#headers()} or
{@link ClientRequestContext#additionalRequestHeaders()}.
core/src/main/java/com/linecorp/armeria/client/AbstractClientOptionsBuilder.java