feat: enhance encoder interface

[shunjiazhu]

Enhance encoder api to support variable keys, i.e. %{metadata.xxx}, ${tag.xxx}, and ${content.xxxx}, just like the converter struct.

converter:

enable: true
global: 
   StrucureType:"v2"
inputs:
  - Type: service_http_server
    Format: influxdb
    Address: "http://127.0.0.1:12345"
    QueryParams:
      - db
aggregators:
  - Type: aggregator_metadata_group
    GroupMetadataKeys:
      - db
flushers:
  - Type: flusher_http
    RemoteURL: "http://127.0.0.1:8086/write"
    Query:
      db: "%{metadata.db}"
    Encoder:
      Type: ext_default_encoder/prometheus