[processor/deltatocumulative] don't copy xsync.MapOf (#39426)

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

xsync.MapOf must not be copied at all, as internal pointer references
become invalid otherwise.

<!-- Issue number (e.g. #1234) or full URL to issue, if applicable. -->
#### Link to tracking issue
Fixes
#39106

#### Testing

Hard to test in a non-flaky way as it depends on high concurrency, but
verified using:

```go
real := xsync.NewMapOf[int64, string]()
copy := *real

ch := make(chan int64, 256)
go func() {
	for i := int64(0); i < math.MaxInt16; i++ {
		copy.LoadOrStore(i, "test")
		ch <- i
	}
	close(ch)
}()

for i := range ch {
	copy.LoadAndDelete(i)
}
```
<!--Please delete paragraphs that you did not use before submitting.-->

Co-authored-by: Antoine Toulme <[email protected]>

Author: tombrk

.chloggen/xsync-ptr.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: deltatocumulative
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: fixes misuse of xsync, leading to bad mutex unlock
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [39106]
+
+# (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: [user]

processor/deltatocumulativeprocessor/internal/maps/map.go

@@ -15,7 +15,7 @@ func Limit(limit int64) Context {
 }
 
 func New[K comparable, V any](ctx Context) *Parallel[K, V] {
-	return &Parallel[K, V]{ctx: ctx, elems: *xsync.NewMapOf[K, V]()}
+	return &Parallel[K, V]{ctx: ctx, elems: xsync.NewMapOf[K, V]()}
 }
 
 // lowercase alias for unexported embedding
@@ -32,7 +32,7 @@ type ctx = Context
 // Parallel enforces the [Context] size limit.
 type Parallel[K comparable, V any] struct {
 	ctx
-	elems xsync.MapOf[K, V]
+	elems *xsync.MapOf[K, V]
 }
 
 // Context holds size information about one or more maps.