fix(protoc-gen-go-http): buildMethodDesc for two and more message field

[nikitaksv]

Description (what this PR does / why we need it):
Generator cannot parse multiple message fields and i fixed it ;)

I have RPC method:

  rpc UpsertLevel (UpsertLevelRequest) returns (UpsertLevelResponse) {
    option (google.api.http) = {
      post: "/v1/organization/{level.organization_id}/levels"
      body: "level"
      response_body: "level"
      additional_bindings: {
        put: "/v1/organizations/{level.organization_id}/levels/{level.id}"
        body: "level"
        response_body: "level"
      }
    };
  }

and message:

message UpsertLevelRequest {
  message Level {
    string id = 1;
    string organization_id = 2;
    // ... and other fields
  }
  Level level = 1;
}

An error occurs during generation: ERROR: The corresponding field 'level.id' declaration in message could not be found in '/v1/organizations/{level.organization_id}/levels/{level.id}'

The error was in the method buildMethodDesc. The "fields" variable overwrote the fields of the child message "message.field".
Now I update the "fields" variable inside the "vars" loop in order to restore the "fields" of the initial message.

[tonybase]

LGTM

[codecov-commenter]

Codecov Report

Merging #1979 (c5c9f95) into main (ddc82ce) will increase coverage by 0.11%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1979      +/-   ##
==========================================
+ Coverage   80.71%   80.82%   +0.11%     
==========================================
  Files          82       82              
  Lines        3635     3635              
==========================================
+ Hits         2934     2938       +4     
+ Misses        494      492       -2     
+ Partials      207      205       -2     

Impacted Files	Coverage Δ
config/file/file.go	66.66% <0.00%> (+4.76%)	⬆️
config/file/watcher.go	65.62% <0.00%> (+6.25%)	⬆️

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