Use flexbox for prometheus graph row grouping instead of bootstrap classes

Replace responsive bootstrap rows/cols and associated logic from prometheus metrics to improve performance, readability, and maintainability.

Before:

<div class="panel-body">
  <div class="prometheus-row row">
    <div class="col-md-6">...</div>
    <div class="col-md-6">...</div>
  </div>
  <div class="prometheus-row row">
    <div class="col-md-12">...</div>
  </div>
</div>

After:

<div class="panel-body prometheus-graph-group">
  <div class="prometheus-graph">...</div>
  <div class="prometheus-graph">...</div>
  <div class="prometheus-graph">...</div>
</div>