fix fieldLimit exception in docLevelMonitor (opensearch-project#1368)

* fix fieldLimit exception in  docLevelMonitor

Signed-off-by: Riya Saxena <[email protected]>

* bug fixes from prev pr

Signed-off-by: Riya Saxena <[email protected]>

---------

Signed-off-by: Riya Saxena <[email protected]>

Author: riysaxen-amzn

alerting/src/main/kotlin/org/opensearch/alerting/DocumentLevelMonitorRunner.kt

@@ -115,8 +115,16 @@ class DocumentLevelMonitorRunner : MonitorRunner() {
             monitorCtx.alertIndices!!.createOrUpdateInitialFindingHistoryIndex(monitor.dataSources)
         } catch (e: Exception) {
             val id = if (monitor.id.trim().isEmpty()) "_na_" else monitor.id
-            logger.error("Error setting up alerts and findings indices for monitor: $id", e)
-            monitorResult = monitorResult.copy(error = AlertingException.wrap(e))
+            val unwrappedException = ExceptionsHelper.unwrapCause(e)
+            if (unwrappedException is IllegalArgumentException && unwrappedException.message?.contains("Limit of total fields") == true) {
+                val errorMessage =
+                    "Monitor [$id] can't process index [$monitor.dataSources] due to field mapping limit"
+                logger.error("Exception: ${unwrappedException.message}")
+                monitorResult = monitorResult.copy(error = AlertingException(errorMessage, RestStatus.INTERNAL_SERVER_ERROR, e))
+            } else {
+                logger.error("Error setting up alerts and findings indices for monitor: $id", e)
+                monitorResult = monitorResult.copy(error = AlertingException.wrap(e))
+            }
         }
         try {
             validate(monitor)