public void logValue(String key, Object value) {
        owner.getModel().getExecution().trace().logValue(key, value.toString());
    }
Adds a key-value which will be logged to the access log for this query (by doing toString() on the value).
Multiple values may be set to the same key. A value cannot be removed once set.
container-search/src/main/java/com/yahoo/search/query/context/QueryContext.java