fix/use `UNSUPPORTED` to represent undefined procotol in labelConvert…

…er (harmonycloud#212)

* fix: avoid to use undefined Protocol

Signed-off-by: niejiangang <niejiangang@harmonycloud.cn>

* fix: add return if exporter is not support trace

Signed-off-by: niejiangang <niejiangang@harmonycloud.cn>

* docs(labelConverter): add comments for `withExtraLabels` function

Signed-off-by: niejiangang <niejiangang@harmonycloud.cn>

* refactor: exact a method for finding extraLabels

Signed-off-by: niejiangang <niejiangang@harmonycloud.cn>