loadbalancer-experimental: make DefaultLoadBalancer the default RR im…

…plementation (#3065)

* loadbalancer-experimental: make DefaultLoadBalancer the default RR implementation

Motivation:

It's been out a while now and DefaultLoadBalancer can exactly behave as
the old RoundRobinLoadBalancer. That means we can insert it in place of
RoundRobinLoadBalancer.

Modifications:

Default to building a DefaultLoadBalancer in round-robin compatibility mode
via the RoundRobinToDefaultLBMigrationProvider. We retain the flag for safety
for a few releases but will eventually remove it and RoundRobinLoadBalancer.

servicetalk-loadbalancer-experimental/src/main/java/io/servicetalk/loadbalancer/RoundRobinToDefaultLBMigrationProvider.java

@@ -53,7 +53,9 @@ RoundRobinLoadBalancerBuilder<ResolvedAddress, C> newBuilder(
     }
 
     private static boolean isEnabled() {
-        return Boolean.getBoolean(PROPERTY_NAME);
+        // Enabled by default.
+        String propValue = System.getProperty(PROPERTY_NAME);
+        return propValue == null || Boolean.parseBoolean(propValue);
     }
 
     private static final class DefaultLoadBalancerRoundRobinBuilder<ResolvedAddress, C extends LoadBalancedConnection>

servicetalk-loadbalancer-experimental/src/test/java/io/servicetalk/loadbalancer/RoundRobinToDefaultLBMigrationProviderTest.java

@@ -61,7 +61,7 @@ void defaultValue() {
                 new RoundRobinLoadBalancerFactory.Builder<>();
         RoundRobinLoadBalancerBuilder<String, TestLoadBalancedConnection> result = provider.newBuilder(
                 "builder", builder);
-        assertThat(result, sameInstance(builder));
+        assertThat(result.build(), instanceOf(DefaultLoadBalancerBuilder.DefaultLoadBalancerFactory.class));
     }
 
     @Test