Limit max embeds to 100

Limits the maximum number of metrics embeds in a single GFM field
to 100 to help with loading. OVer 100 has a hard time loading.

changelogs/unreleased/sy-add-embeds-limit.yml

+---
+title: Limit max metrics embeds in GFM to 100
+merge_request: 21356
+author:
+type: performance

doc/user/project/integrations/prometheus.md

@@ -476,7 +476,7 @@ Prometheus server.
 
 > [Introduced][ce-29691] in GitLab 12.2.
 
-It is possible to display metrics charts within [GitLab Flavored Markdown](../../markdown.md#gitlab-flavored-markdown-gfm).
+It is possible to display metrics charts within [GitLab Flavored Markdown](../../markdown.md#gitlab-flavored-markdown-gfm). The maximum number of embeds allowed in a GitLab Flavored Markdown field is 100.
 
 NOTE: **Note:**
 Requires [Kubernetes](prometheus_library/kubernetes.md) metrics.

lib/banzai/filter/inline_metrics_redactor_filter.rb

@@ -8,6 +8,7 @@ module Banzai
       include Gitlab::Utils::StrongMemoize
 
       METRICS_CSS_CLASS = '.js-render-metrics'
+      EMBED_LIMIT = 100
       URL = Gitlab::Metrics::Dashboard::Url
 
       Embed = Struct.new(:project_path, :permission)
@@ -35,9 +36,16 @@ module Banzai
       # Returns all nodes which the FE will identify as
       # a metrics embed placeholder element
       #
+      # Removes any nodes beyond the first 100
+      #
       # @return [Nokogiri::XML::NodeSet]
       def nodes
-        @nodes ||= doc.css(METRICS_CSS_CLASS)
+        strong_memoize(:nodes) do
+          nodes = doc.css(METRICS_CSS_CLASS)
+          nodes.drop(EMBED_LIMIT).each(&:remove)
+
+          nodes
+        end
       end
 
       # Maps a node to key properties of an embed.

spec/lib/banzai/filter/inline_metrics_redactor_filter_spec.rb

@@ -55,11 +55,29 @@ describe Banzai::Filter::InlineMetricsRedactorFilter do
       it_behaves_like 'a supported metrics dashboard url'
     end
 
-    context 'for an internal non-dashboard url' do
-      let(:url) { urls.project_url(project) }
+    context 'the user has requisite permissions' do
+      let(:user) { create(:user) }
+      let(:doc) { filter(input, current_user: user) }
 
-      it 'leaves the placeholder' do
-        expect(doc.to_s).to be_empty
+      before do
+        project.add_maintainer(user)
+      end
+
+      context 'for an internal non-dashboard url' do
+        let(:url) { urls.project_url(project) }
+
+        it 'leaves the placeholder' do
+          expect(doc.to_s).to be_empty
+        end
+      end
+
+      context 'with over 100 embeds' do
+        let(:embed) { %(<div class="js-render-metrics" data-dashboard-url="#{url}"></div>) }
+        let(:input) { embed * 150 }
+
+        it 'redacts ill-advised embeds' do
+          expect(doc.to_s.length).to eq(embed.length * 100)
+        end
       end
     end
   end