add config description for opentracing (#8392)
add config description for opentracing (#8392)

* add config description for opentracing

* Update tidb-configuration-file.md

Co-authored-by: Enwei <01d6a0ae30b38a2011b1a46fe95012dcde208d7f@pingcap.com>

Co-authored-by: Enwei <01d6a0ae30b38a2011b1a46fe95012dcde208d7f@pingcap.com>
pingcap/docs,pingcap/docs,pingcap/docs