Include idempotency level in UnaryServerInfo

[keelerh]

Adding idempotency level information to the UnaryServerInfo allows idempotency level to be used in server interceptors.

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[menghanl]

Can you explain a bit more on the purpose of this change?
Also, IdempotencyLevel is never set to non-default value in the change.

[keelerh]

Protoc now supports an idempotency_levelmethod option: https://github.com/google/protobuf/blob/9e745f7/src/google/protobuf/descriptor.proto#L630. The method option can be used to mark the idempotency of a gRPC method.

service Greeter {
  // Sends a greeting
  rpc SayHello (HelloRequest) returns (HelloReply) {
    option idempotency_level = NO_SIDE_EFFECTS;
  }
}

IdempotencyLevel would be set to a non-default value by explicitly setting the idempotency_level method option for a given service.

The purpose of the change is to include this idempotency_level information with the previously defined UnaryServerInfo such that it can be used at a later point, e.g. by server-side gRPC interceptors designed to ensure that services marked as idempotent do not suddenly become non-idempotent by calling a non-idempotent method.

[googlebot]

CLAs look good, thanks!

[keelerh]

I signed it!

[dfawley]

Thanks for the PR. We would prefer to implement this by making the entire proto file descriptor available to interceptors, which would include this information. (#1526) Unfortunately, that requires changes to generated code, which means it's blocked on us (this PR would have the same constraint). We hope to get to this early next quarter. Please let us know if you have any concerns with this.