synchronized void succeed(Upstream upstream) {
        Objects.requireNonNull(upstream);
        UpstreamState upstreamState = findUpstreamState(upstream);
        upstreamState.succeeded();
    }
Tiered load balancer. Each next() call returns an available upstream from the highest tier (lowest tier value in config).
If the whole tier (highest) is unavailable, balancer start routing upstreams from next tier (lower) if any.
server/src/main/java/com/epam/aidial/core/server/upstream/TieredBalancer.java