@Nullable
    default Long defaultPingIntervalMillis() {
        return null;
    }
Returns the default value for the PING interval.
A <a href="https://datatracker.ietf.org/doc/html/rfc7540#section-6.7">PING</a> frame
is sent for HTTP/2 server and client 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 client.

<p>Note that this flag is only in effect when {@link #defaultServerIdleTimeoutMillis()} for server and
{@link #defaultClientIdleTimeoutMillis()} for client are greater than the value of this flag.

<p>The default value of this flag is {@value DefaultFlagsProvider#DEFAULT_PING_INTERVAL_MILLIS}
milliseconds. Specify the {@code -Dcom.linecorp.armeria.defaultPingIntervalMillis=<integer>} JVM option
to override the default value. If the specified value was smaller than 10 seconds, bumps PING
interval to 10 seconds.
core/src/main/java/com/linecorp/armeria/common/FlagsProvider.java