metadataMarkMPObjectForCompletion(params, log, cb) {
        assert.strictEqual(typeof params, 'object');
        assert.strictEqual(typeof params.bucketName, 'string');
        assert.strictEqual(typeof params.objectKey, 'string');
        assert.strictEqual(typeof params.uploadId, 'string');
        assert.strictEqual(typeof params.splitter, 'string');
        assert.strictEqual(typeof params.storedMetadata, 'object');
        const splitter = params.splitter;
        const longMPUIdentifier =
            `overview${splitter}${params.objectKey}${splitter}${params.uploadId}`;
        const multipartObjectMD = Object.assign({}, params.storedMetadata);
        multipartObjectMD.completeInProgress = true;
        metadata.putObjectMD(params.bucketName, longMPUIdentifier, multipartObjectMD,
        {}, log, err => {
            if (err) {
                log.error('error from metadata', { error: err });
                return cb(err);
            }
            return cb();
        });
    }
Mark the MPU overview key with a flag when starting the
CompleteMPU operation, to be checked by "put part" operations

@param {object} params - params object
@param {string} params.bucketName - name of MPU bucket
@param {string} params.objectKey - object key
@param {string} params.uploadId - upload ID
@param {string} params.splitter - splitter for this overview key
@param {object} params.storedMetadata - original metadata of the overview key
@param {Logger} log - Logger object
@param {function} cb - callback(err)
@return {undefined}