Census: enable requestz sampling for generated methods

[zhangkun83]

Census has an open-source requestz implementation that can show traces of live, sampled and error RPCs. In order to sample RPCs, gRPC needs to call SampledSpanStore.registerSpanNamesForCollection() with any known method name. This is fairly expensive, thus should not be called on a per-RPC basis. A few options:

1. In a static initializer in the generated code. If we don't want to call Census directly from the generated code, maybe call Census code in a helper function in ClientCalls and ServerCalls which is called from generated code.
2. In MethodDescriptor constructor. This will be called once per method, which might slow start-up speed. This would also all custom-created methods to be registered, @bogdandrutu is this desirable, or do you only expect generated methods to be registered?

Option 1 sounds like a better option. @ejona86 WDYT?

[zhangkun83]

#3471 was reverted in 1.7.x because we were not happy about the global registry. We will migrate to a new Census API that allows us to specify requestz sampling on a per-RPC basis.