Geo: Gracefully handle a non-JSON response from the node status

See https://gitlab.com/gitlab-org/gitlab-ee/issues/5993

ee/app/services/geo/node_status_fetch_service.rb

@@ -11,7 +11,11 @@ module Geo
         data[:success] = response.success?
 
         if response.success?
-          data.merge!(response.parsed_response)
+          if response.parsed_response.is_a?(Hash)
+            data.merge!(response.parsed_response)
+          else
+            data[:health] = 'A JSON response was not received'
+          end
         else
           message = "Could not connect to Geo node - HTTP Status Code: #{response.code} #{response.message}"
           payload = response.parsed_response

ee/changelogs/unreleased/sh-geo-handle-invalid-json-response.yml

+---
+title: 'Geo: Gracefully handle a non-JSON response from the node status'
+merge_request:
+author:
+type: fixed

ee/spec/services/geo/node_status_fetch_service_spec.rb

@@ -79,6 +79,18 @@ describe Geo::NodeStatusFetchService, :geo do
       expect(status.success).to be true
     end
 
+    it 'handles invalid JSON response' do
+      request = double(success?: true,
+                       code: 200,
+                       message: 'Something here',
+                       parsed_response: 'Something here')
+      allow(Gitlab::HTTP).to receive(:get).and_return(request)
+
+      status = subject.call(secondary)
+
+      expect(status.status_message).to eq("A JSON response was not received")
+    end
+
     it 'omits full response text in status' do
       request = double(success?: false,
                        code: 401,