public long pingIntervalMillis() {
        return get(PING_INTERVAL_MILLIS);
    }
Returns the PING interval in milliseconds.
When neither read nor write was performed for the specified period of time,
a <a href="https://datatracker.ietf.org/doc/html/rfc7540#section-6.7">PING</a> frame is sent for HTTP/2
or an <a href="https://datatracker.ietf.org/doc/html/rfc7231#section-4.3.7">OPTIONS</a> request with
an asterisk ("*") is sent for HTTP/1.
core/src/main/java/com/linecorp/armeria/client/ClientFactoryOptions.java