private void setPendingAcquisition(SessionProtocol desiredProtocol, PoolKey key,
                                       ChannelAcquisitionFuture future) {
        pendingAcquisitions[desiredProtocol.ordinal()].put(key, future);
    }
Returns an array whose index signifies {@link SessionProtocol#ordinal()}. Similar to {@link EnumMap}.
core/src/main/java/com/linecorp/armeria/client/HttpChannelPool.java