Add mem_shared to Linux schema validation

- Added mem_shared to validLinuxMetrics.json for schema validation
- Ensures Linux-specific memory metric is accepted by configuration schema

Author: Ola Amr

translator/config/sampleSchema/validLinuxMetrics.json

@@ -62,6 +62,7 @@
         "measurement": [
           "mem_used",
           "mem_cached",
+          "mem_shared",
           "mem_total"
         ],
         "metrics_collection_interval": 1