stats: make easier to reuse stats code

[wbpcode]

Commit Message: stats: make easier to reuse stats code
Additional Description:

1. Peter tags span rather than reference to vector for metrics creation. By this way, we can use inlined vector, array and so on rather than only vector.
2. minor refactor to make it's easier to reuse stats code if we want another custom scope implemenation.

No public code API change and won't affect our forks.

Risk Level:
Testing:
Docs Changes:
Release Notes:
Platform Specific Features:
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[Optional Fixes #Issue]
[Optional Fixes commit #PR or SHA]
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[wbpcode]

/retest