Fix null-safety in DefaultMongodbObservationConvention and MongodbObservationContext

Author: nhachicha

driver-core/src/main/com/mongodb/observability/micrometer/DefaultMongodbObservationConvention.java

@@ -155,7 +155,7 @@ private KeyValues getCommandHighCardinalityKeyValues(final MongodbObservationCon
         Throwable error = context.getError();
         if (error != null) {
             kv = kv.and(
-                    MongodbObservation.HighCardinalityKeyNames.EXCEPTION_MESSAGE.withValue(error.getMessage()),
+                    MongodbObservation.HighCardinalityKeyNames.EXCEPTION_MESSAGE.withValue(String.valueOf(error.getMessage())),
                     MongodbObservation.HighCardinalityKeyNames.EXCEPTION_TYPE.withValue(error.getClass().getName()),
                     MongodbObservation.HighCardinalityKeyNames.EXCEPTION_STACKTRACE.withValue(getStackTraceAsString(error)));
         }

driver-core/src/main/com/mongodb/observability/micrometer/MongodbObservationContext.java

@@ -115,6 +115,7 @@ public void setConnectionId(@Nullable final ConnectionId connectionId) {
         this.connectionId = connectionId;
     }
 
+    @Nullable
     public MongodbObservation getObservationType() {
         return observationType;
     }