use header field and value as subset lb key value directly

[wbpcode]

Title: use header field and value as subset lb key value directly

The subset lb provides great feature to match a subset of cluster's endpoints.

But it require us to set the static metadata_match (this feature make limited sense, because when we use the subset lb, we may also expect dynamic input) or set the dynamic metadata by a filter (like header_to_metadata).

I think it would be much simpler for users to use the headers as the key/value pairs input directly to match dynamic subset.