fix: Correctly interpret x-request-start header when measured in nano…

…seconds (#64)

judoscale-ruby/lib/judoscale/request_metrics.rb

@@ -18,11 +18,18 @@ def ignore?
 
     def started_at
       if @request_start_header
-        # Heroku sets the header as an integer, measured in milliseconds.
-        # If nginx is involved, it might be in seconds with fractional milliseconds,
-        # and it might be preceeded by "t=". We can all cases by removing non-digits
-        # and treating as milliseconds.
-        Time.at(@request_start_header.gsub(/\D/, "").to_i / 1000.0)
+        # There are several variants of this header. We handle these:
+        #   - whole milliseconds (Heroku)
+        #   - whole nanoseconds (Render)
+        #   - fractional seconds (NGINX)
+        #   - preceeding "t=" (NGINX)
+        value = @request_start_header.gsub(/[^0-9.]/, "").to_f
+
+        case value
+        when 0..100_000_000_000 then Time.at(value)
+        when 100_000_000_000..100_000_000_000_000 then Time.at(value / 1000.0)
+        else Time.at(value / 1_000_000.0)
+        end
       end
     end
 

judoscale-ruby/test/request_metrics_test.rb

@@ -33,6 +33,18 @@ module Judoscale
         end
       end
 
+      it "handles X_REQUEST_START in nanoseconds" do
+        freeze_time do
+          started_at = Time.now.utc - 2
+          ended_at = started_at + 1
+          env["HTTP_X_REQUEST_START"] = (started_at.to_f * 1_000_000).to_i.to_s
+
+          # The queue time might return 999 or 1000 depending to time + float + format conversion,
+          # even when freezing time.
+          _(request.queue_time(ended_at)).must_be_within_delta 1000, 1
+        end
+      end
+
       it "subtracts the network time / request body wait available in puma from the queue time" do
         freeze_time do
           started_at = Time.now.utc - 2