samplegen maximum recursion error when recursive message in oneof

[dizcology]

To reproduce with version 1.11.9:

1. Add the following to tests/fragments:

# tests/fragments/test_recursive_messages_oneof.proto

syntax = "proto3";

package google.fragment;

import "google/api/client.proto";

service MyService {
  option (google.api.default_host) = "my.example.com";

  rpc MyMethod(MethodRequest) returns (MethodResponse) {}
}

message MethodRequest {
  oneof request {
    MethodRequest child = 1;
    string data = 2;
  }
}

message MethodResponse {}

2. Run nox -s fragment-3.7 -- tests/fragments/test_recursive_messages_oneof.proto.
3. The test fails with RecursionError: maximum recursion depth exceeded while calling a Python object. The recursion happens at this line: https://github.com/googleapis/gapic-generator-python/blob/v1.11.9/gapic/samplegen/samplegen.py#L992, around which the previous maintainer of samplegen.py included a warning and a possible solution.

The error only occurs when there is a oneof whose first field is recursive. Variants of the fragment proto that do not result in the same error:

1. If there is no oneof.
2. If there is no recursion.
3. If there are mutual/multiple recursions where at least one recursion is not the first field in a oneof.
4. If the recursion does not occur in the first field belonging to the oneof. Here "first" refers to the proto file's represenation, and not the field ID number.

[dizcology]

Another way the same issue occurs is when a recursive message is marked REQUIRED:

# tests/fragments/test_recursive_messages_required.proto

syntax = "proto3";

package google.fragment;

import "google/api/client.proto";
import "google/api/field_behavior.proto";

service MyService {
  option (google.api.default_host) = "my.example.com";

  rpc MyMethod(MethodRequest) returns (MethodResponse) {}
}

message MethodRequest {
  MethodRequest child = 1 [(google.api.field_behavior) = REQUIRED];
}

message MethodResponse {}

To be sure, this proto fragment is unusable since a MethodRequest message (that conforms to the field_behavior) can never be constructed.

In both cases (oneof and REQUIRED), the issue is that the same message type could recursively appear in request_fields (https://github.com/googleapis/gapic-generator-python/blob/v1.11.11/gapic/samplegen/samplegen.py#L965). The fix should be to exclude message types that have already appeared when populating the sample request message used in the generated snippets.