[processor/deltatocumulativeprocessor]: fix incorrect bucket counts when downscaling exp histograms (#33831)

**Description:** <Describe what has changed.>
<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->

The downscaling algorithm collapses bucket by pairing neighbors into new
buckets. The simple case occurs when there is an even number of buckets
and offset is zero or even.
```
1 1 1 1
 2   2
```

When there is an odd number of buckets, the last bucket is accounted for
by incrementing the `size` variable to ensure an extra loop iteration.
```
1 1 1
 2   1
```

When offset is odd, the first bucket is accounted for by special
handling in the loop.
```
ø 1 1 1
 1   2
```

However, when there is an even number of buckets, AND an odd offset,
this happens:
```
ø 1 1
 1   0
```

The collapsed buckets are incorrect because the buckets sum up to 1
(1+0) instead of 2 (1+1).

Handle this edge case by incrementing the `size` variable when offset is
odd
```
ø 1 1
 1   1
```

**Link to tracking Issue:** <Issue number if applicable>

**Testing:** <Describe what testing was performed and which tests were
added.>

Added some test cases to showcase the bug.

**Documentation:** <Describe the documentation added.>

Author: edma2

.chloggen/fix-deltatocumulative-nh-downscaling.yaml

@@ -0,0 +1,27 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: bug_fix
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: deltatocumulativeprocessor
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: fix bucket counts when downscaling exp histograms with odd offsets
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [33831]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: []

processor/deltatocumulativeprocessor/internal/data/expo/scale.go

@@ -72,7 +72,7 @@ func Downscale(bs Buckets, from, to Scale) {
 func Collapse(bs Buckets) {
 	counts := bs.BucketCounts()
 	size := counts.Len() / 2
-	if counts.Len()%2 != 0 {
+	if counts.Len()%2 != 0 || bs.Offset()%2 != 0 {
 		size++
 	}
 

processor/deltatocumulativeprocessor/internal/data/expo/scale_test.go

@@ -45,6 +45,18 @@ func TestDownscale(t *testing.T) {
 	}, {
 		{scale: 2, bkt: "1 1 1 1 1 1 1 1 1 1 1 1"},
 		{scale: 0, bkt: "   4       4       4   "},
+	}, {
+		{scale: 1, bkt: "ø 1 1 0"},
+		{scale: 0, bkt: " 1   1 "},
+	}, {
+		{scale: 1, bkt: "ø 1 1 "},
+		{scale: 0, bkt: " 1   1"},
+	}, {
+		{scale: 1, bkt: " - 1 1 "},
+		{scale: 0, bkt: "- 1   1"},
+	}, {
+		{scale: 5, bkt: "-  4 0 3 0 3 0 0 8   "},
+		{scale: 4, bkt: "- 4   3   3   0   8  "},
 	}}
 
 	type B = expo.Buckets
@@ -58,6 +70,10 @@ func TestDownscale(t *testing.T) {
 						bkt.SetOffset(bkt.Offset() + 1)
 						continue
 					}
+					if elem == "-" {
+						bkt.SetOffset(bkt.Offset() - 1)
+						continue
+					}
 					n, err := strconv.Atoi(elem)
 					if err != nil {
 						panic(err)