[chore] add missing stability for es receiver metric (open-telemetry#43899)

This patch adds the missing stability definition for the
`elasticsearch.node.open_files` metric.

Spotted at
open-telemetry/opentelemetry-collector#13920.

Signed-off-by: ChrsMark <[email protected]>

Author: ChrsMark

receiver/elasticsearchreceiver/documentation.md

@@ -464,9 +464,9 @@ Total number of failed ingest operations during the lifetime of this node.
 
 The number of open file descriptors held by the node.
 
-| Unit | Metric Type | Value Type | Aggregation Temporality | Monotonic |
-| ---- | ----------- | ---------- | ----------------------- | --------- |
-| {files} | Sum | Int | Cumulative | false |
+| Unit | Metric Type | Value Type | Aggregation Temporality | Monotonic | Stability |
+| ---- | ----------- | ---------- | ----------------------- | --------- | --------- |
+| {files} | Sum | Int | Cumulative | false | development |
 
 ### elasticsearch.node.operations.completed
 

receiver/elasticsearchreceiver/metadata.yaml

@@ -753,6 +753,8 @@ metrics:
     enabled: true
   elasticsearch.node.open_files:
     description: The number of open file descriptors held by the node.
+    stability:
+      level: development
     unit: "{files}"
     sum:
       monotonic: false