title: Firestore REST API starting query
body: <p>I am struggling with Firestore REST API. Database have a collection with Users. Every user is separate document. I need to get all users with, for example, category equal 1. Here is my attempt (request body): </p>
  
  <pre><code>{
        "structuredQuery": {
              "where" : {
                  "fieldFilter" : { 
                        "field": { "fieldPath" : "category" } , 
                                   "op":"EQUAL", 
                                   "value": { "integerValue" : 1 }
                                  }
                        }
  
        }
  
      }
  </code></pre>
  
  <p>and response error: </p>
  
  <pre><code>"error": {
    "code": 400,
    "message": "kind is required for filter: category",
    "status": "INVALID_ARGUMENT"
  }
  </code></pre>
  
  <p>I have totaly no idea, how "field" inside "fieldFilter" should looks like. Thanks in advance. </p>