pluginfw: Hook to include additional body in HTML exports (#4469)

* hook, needs docs

* docs

doc/api/hooks_server-side.md

@@ -661,6 +661,24 @@ function _analyzeLine(alineAttrs, apool) {
 }
 ```
 
+## exportHTMLAdditionalContent 
+Called from: src/node/utils/ExportHtml.js
+
+Things in context:
+
+1. padId
+
+This hook will allow a plug-in developer to include additional HTML content in the Body of the exported HTML
+
+Example:
+```
+exports.exportHTMLAdditionalContent = async (hookName, context) => {
+  let padId = context;
+  return "I am groot in " + padId;
+}
+
+```
+
 ## stylesForExport
 Called from: src/node/utils/ExportHtml.js
 

src/node/utils/ExportHtml.js

@@ -498,6 +498,11 @@ exports.getPadHTMLDocument = async function (padId, revNum)
 
   let html = await getPadHTML(pad, revNum);
 
+  let exportHTMLAdditionalContent = await hooks.aCallAll("exportHTMLAdditionalContent", padId);
+  exportHTMLAdditionalContent.forEach(function(hookHtml){
+    html += hookHtml;
+  });
+
   return eejs.require("ep_etherpad-lite/templates/export_html.html", {
     body: html,
     padId: Security.escapeHTML(padId),