[exporter/datadog] Fix data race in metrics exporter shutdown (#31838)

**Description:** 
Fix data race in metrics exporter shutdown.
If we do not do `f.wg.Wait()` before closing `statsToAgent`, there can
be a race condition when the goroutine still tries to write to
`statsToAgent` after it is closed.

Fixes
#31663

.chloggen/ddog-data-race.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: datadogexporter
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Fix data race in metrics exporter shutdown
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [31663]
+
+# (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: []

exporter/datadogexporter/factory.go

@@ -355,7 +355,8 @@ func (f *factory) createMetricsExporter(
 		exporterhelper.WithCapabilities(consumer.Capabilities{MutatesData: true}),
 		exporterhelper.WithQueue(cfg.QueueSettings),
 		exporterhelper.WithShutdown(func(context.Context) error {
-			cancel()
+			cancel()    // first cancel context
+			f.wg.Wait() // then wait for shutdown
 			f.StopReporter()
 			statsWriter.Stop()
 			if statsIn != nil {