chore: add NullNode case while converting values for mertrics reporting (#222)

Author: sanket-mundra

document-store/src/main/java/org/hypertrace/core/documentstore/metric/BaseDocStoreMetricProviderImpl.java

@@ -140,6 +140,8 @@ private String getStringLabelValue(JsonNode jsonNode) {
         return String.valueOf(jsonNode.numberValue());
       case BOOLEAN:
         return String.valueOf(jsonNode.booleanValue());
+      case NULL:
+        return NULL_LABEL_VALUE_PLACEHOLDER;
       default:
         throw new IllegalArgumentException(
             String.format("Unsupported JSON node type: %s", jsonNode.getNodeType()));