[exporter/datadog] Set Capabilities on metrics exporter (#24979)

**Description:** 

Correctly set `MutatesData` to `true` on the Datadog metrics exporter.
This was leading to panics when using multiple exporters.

**Link to tracking Issue:** Fixes #24908

Author: mx-psi

.chloggen/mx-psi_fix-panics.yaml

@@ -0,0 +1,28 @@
+# 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: Correctly set metrics exporter capabilities to state that it mutates data
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [24908]
+
+# (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: | 
+  This could lead to random panics if using more than one metrics exporter.
+
+# 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

@@ -275,6 +275,8 @@ func (f *factory) createMetricsExporter(
 		exporterhelper.WithTimeout(exporterhelper.TimeoutSettings{Timeout: 0 * time.Second}),
 		// We use our own custom mechanism for retries, since we hit several endpoints.
 		exporterhelper.WithRetry(exporterhelper.RetrySettings{Enabled: false}),
+		// The metrics remapping code mutates data
+		exporterhelper.WithCapabilities(consumer.Capabilities{MutatesData: true}),
 		exporterhelper.WithQueue(cfg.QueueSettings),
 		exporterhelper.WithShutdown(func(context.Context) error {
 			cancel()