title: histogram_quantile isn't plotting correctly for multiple labels
body: <p>I tried to plot using <code>histogram_quantile</code> with Prometheus with sum on two labels, i.e.</p>
  
  <p><code>histogram_quantile(0.9, sum(rate(grpc_request_duration_seconds_bucket{service="$service"}[$interval])) by (le, grpc_method))</code></p>
  
  <p>The resulting plot was not the same as me manually making a separate plot for each individual value of <code>grpc_method</code>:</p>
  
  <p><code>histogram_quantile(0.9, sum(rate(grpc_request_duration_seconds_bucket{service="$service",grpc_method="GetGridFile"}[$interval])) by (le, grpc_method))</code></p>
  
  <p>and</p>
  
  <p><code>histogram_quantile(0.9, sum(rate(grpc_request_duration_seconds_bucket{service="$service",grpc_method="PostAgvState"}[$interval])) by (le, grpc_method))</code></p>
  
  <p><a href="https://i.stack.imgur.com/xrmF0.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/xrmF0.png" alt="enter image description here"></a></p>