FIX: Don't raise error for custom metrics while converting to hash

FIX: Don’t raise error for custom metrics while converting to hash

diff --git a/lib/internal_metric/base.rb b/lib/internal_metric/base.rb
index 9f1faa8..c876770 100644
--- a/lib/internal_metric/base.rb
+++ b/lib/internal_metric/base.rb
@@ -61,6 +61,8 @@ module DiscoursePrometheus::InternalMetric
           "Web"
         when Process
           "Process"
+        when Custom
+          "Custom"
         else
           raise "not implemented"
         end

GitHub sha: 3731c713

Create rspec test case for existing fix