title: Azure Cosmos DB: 'Request rate is large' for simple count query
body: <p>I'm using Cosmos DB with the Mongo adapter, accessing via the <a href="https://github.com/mongodb/mongo-ruby-driver" rel="noreferrer">Ruby mongo driver</a>. Currently there are about 2.5M records in the db.</p>
  
  <p>When querying the total amount of records, there's no problem whatsoever:</p>
  
  <pre><code>2.2.5 :011 &gt; mongo_collection.count
  D, [2017-11-24T11:52:39.796716 #9792] DEBUG -- : MONGODB | XXX.documents.azure.com:10255 | admin.count | STARTED | {"count"=&gt;"xp_events", "query"=&gt;{}}
  D, [2017-11-24T11:52:39.954645 #9792] DEBUG -- : MONGODB | XXX.documents.azure.com:10255 | admin.count | SUCCEEDED | 0.15778699999999998s
   =&gt; 2565825
  </code></pre>
  
  <p>But when I try to count the amount of records found based on a simple where, I run into the <code>Request rate is large</code> error:</p>
  
  <pre><code>2.2.5 :014 &gt; mongo_collection.find(some_field: 'some_value').count
  D, [2017-11-24T11:56:11.926812 #9792] DEBUG -- : MONGODB | XXX.documents.azure.com:10255 | admin.count | STARTED | {"count"=&gt;"some_table", "query"=&gt;{"some_field"=&gt;"some_value"}}
  D, [2017-11-24T11:56:24.629659 #9792] DEBUG -- : MONGODB | XXX.documents.azure.com:10255 | admin.count | FAILED | Message: {"Errors":["Request rate is large"]}
  ActivityId: 0000-0000-0000-000000000000, Request URI: /apps/XXX/services/XXX/partitions/XXX/replicas/XXX/, RequestStats: , SDK: Microsoft.Azure.Documents.Common/1.17.101.1 (16500), Message: {"Errors":["Request rate is large"]}
  </code></pre>
  
  <p>I understand how the error works, but I don't understand how such a query can max out the RU/s (set at max value 10,000), since the field I'm querying is supposed to be indexed (automatically).</p>
  
  <p>Any advice would be greatly appreciated!</p>