private void filterMetadataList(FetchEmitTuple t, MetadataListAndEmbeddedBytes parseData) {
        MetadataListFilter filter = t.getParseContext().get(MetadataListFilter.class);
        if (filter == null) {
            filter = tikaConfig.getMetadataListFilter();
        }
        if (filter instanceof NoOpListFilter) {
            return;
        }
        try {
            parseData.filter(filter);
        } catch (TikaException e) {
            LOG.warn("failed to filter metadata list", e);
        }
    }
returns stack trace if there was a container exception or empty string
if there was no stacktrace

@param t
@param metadataList
@return
tika-core/src/main/java/org/apache/tika/pipes/PipesServer.java