Fix MetricEvent timestamp serialization to float

Author: sl0thentr0py

sentry-ruby/lib/sentry/metric_event.rb

@@ -33,7 +33,7 @@ def to_h
         type: @type,
         value: @value,
         unit: @unit,
-        timestamp: @timestamp,
+        timestamp: @timestamp.to_f,
         trace_id: @trace_id,
         span_id: @span_id,
         attributes: serialize_attributes

sentry-ruby/spec/sentry/metric_event_spec.rb

@@ -59,7 +59,7 @@
       expect(hash[:type]).to eq(:distribution)
       expect(hash[:value]).to eq(5.0)
       expect(hash[:unit]).to eq("seconds")
-      expect(hash[:timestamp]).to be_a(Time)
+      expect(hash[:timestamp]).to be_a(Float)
     end
 
     it "includes trace info if provided" do