Expose metrics element for FE consumption

Adds GFM Pipline filters to insert a placeholder in the generated
HTML from GFM based on the presence of a metrics dashboard link.

The front end should look for the class 'js-render-metrics' to
determine if it should replace the element with metrics charts.
The data element 'data-dashboard-url' should be the endpoint
the front end should hit in order to obtain a dashboard layout
in order to appropriately render the charts.

1 files changed, 67 insertions, 0 deletions

diff --git a/lib/banzai/filter/inline_embeds_filter.rb b/lib/banzai/filter/inline_embeds_filter.rb
new file mode 100644
index 00000000000..97394fd8f82
--- /dev/null
+++ b/lib/banzai/filter/inline_embeds_filter.rb
@@ -0,0 +1,67 @@
+# frozen_string_literal: true
+
+module Banzai
+  module Filter
+    # HTML filter that inserts a node for each occurence of
+    # a given link format. To transform references to DB
+    # resources in place, prefer to inherit from AbstractReferenceFilter.
+    class InlineEmbedsFilter < HTML::Pipeline::Filter
+      # Find every relevant link, create a new node based on
+      # the link, and insert this node after any html content
+      # surrounding the link.
+      def call
+        return doc unless Feature.enabled?(:gfm_embedded_metrics, context[:project])
+
+        doc.xpath(xpath_search).each do |node|
+          next unless element = element_to_embed(node)
+
+          # We want this to follow any surrounding content. For example,
+          # if a link is inline in a paragraph.
+          node.parent.children.last.add_next_sibling(element)
+        end
+
+        doc
+      end
+
+      # Implement in child class.
+      #
+      # Return a Nokogiri::XML::Element to embed in the
+      # markdown.
+      def create_element(params)
+      end
+
+      # Implement in child class unless overriding #embed_params
+      #
+      # Returns the regex pattern used to filter
+      # to only matching urls.
+      def link_pattern
+      end
+
+      # Returns the xpath query string used to select nodes
+      # from the html document on which the embed is based.
+      #
+      # Override to select nodes other than links.
+      def xpath_search
+        'descendant-or-self::a[@href]'
+      end
+
+      # Creates a new element based on the parameters
+      # obtained from the target link
+      def element_to_embed(node)
+        return unless params = embed_params(node)
+
+        create_element(params)
+      end
+
+      # Returns a hash of named parameters based on the
+      # provided regex with string keys.
+      #
+      # Override to select nodes other than links.
+      def embed_params(node)
+        url = node['href']
+
+        link_pattern.match(url) { |m| m.named_captures }
+      end
+    end
+  end
+end