fix(prometheus) Metric name should not be set twice

[amolnayak311]

Pull Request checklist

• [x] The commit(s) message(s) follows the contribution guidelines ?
• [x] Tests for the changes have been added (for bug fixes / features) ?
• [ ] Docs have been added / updated (for bug fixes / features) ?

Current behavior : Following prom expressions are valid

foo{_metric_="foo"} but foo{__name__="foo"} will fail to parse

New behavior : both foo{_metric_="foo"} and foo{__name__="foo"} will fail to parse