xds: Circuit breaking doesn't support unsigned 32 bits

[ejona86]

If the control plane sends MAX_UINT32 as the max_requests for circuit breaking, grpc-java will treat it as a negative number. That should cause circuit breaking to trigger for all RPCs.

max_requests is a UInt32Value, but that's encoded as a signed int in Java. The code casts to long, but the sign is preserved:

grpc-java/xds/src/main/java/io/grpc/xds/XdsClusterResource.java

Line 216 in 4e8f7df

maxConcurrentRequests = (long) threshold.getMaxRequests().getValue();

To treat it as a uint32, you essentially need to AND it with 0xFFFFFFFFL to make it unsigned. Since this is in xDS, we can use Integer.toUnsignedLong(int) (the main other convenience being Guava's UnsignedInts.toLong(int)).

b/372943501

CC @kannanjgithub