fix: use mapping scale outside rescale logic (#1891)

* fix: use mapping scale

* add test case

---------

Co-authored-by: Kayla Reopelle <[email protected]>

Author: xuan-cao-swi

metrics_sdk/lib/opentelemetry/sdk/metrics/aggregation/exponential_bucket_histogram.rb

@@ -140,13 +140,14 @@ def update(amount, attributes, data_points)
               scale_change = get_scale_change(low, high)
               downscale(scale_change, hdp.positive, hdp.negative)
               new_scale = @mapping.scale - scale_change
-              hdp.scale = new_scale
               @mapping = new_mapping(new_scale)
               bucket_index = @mapping.map_to_index(amount)
 
               OpenTelemetry.logger.debug "Rescaled with new scale #{new_scale} from #{low} and #{high}; bucket_index is updated to #{bucket_index}"
             end
 
+            hdp.scale = @mapping.scale
+
             # adjust buckets based on the bucket_index
             if bucket_index < buckets.index_start
               span = buckets.index_end - bucket_index

metrics_sdk/test/opentelemetry/sdk/metrics/aggregation/exponential_bucket_histogram_test.rb

@@ -136,6 +136,28 @@
       _(exphdps[0].zero_threshold).must_equal(0)
     end
 
+    it 'adjusts_scale_after_initial_zero_value' do
+      expbh = OpenTelemetry::SDK::Metrics::Aggregation::ExponentialBucketHistogram.new(
+        aggregation_temporality: aggregation_temporality,
+        record_min_max: record_min_max,
+        zero_threshold: 0
+      )
+
+      expbh.update(0, {}, data_points)
+      expbh.update(10_000, {}, data_points)
+
+      exphdps = expbh.collect(start_time, end_time, data_points)
+
+      _(exphdps.size).must_equal(1)
+      _(exphdps[0].count).must_equal(2)
+      _(exphdps[0].sum).must_equal(10_000)
+      _(exphdps[0].min).must_equal(0)
+      _(exphdps[0].max).must_equal(10_000)
+      _(exphdps[0].scale).must_equal(20)
+      _(exphdps[0].zero_count).must_equal(1)
+      _(exphdps[0].zero_threshold).must_equal(0)
+    end
+
     it 'test_permutations' do
       test_cases = [
         [