Tracking issue: support Loki write

[shuiyisong]

What problem does the new feature solve?

The ability for GreptimeDB to serve as a remote write backend for Loki push requests.

What does the feature do?

TODO

• Loki protobuf write path: feat: Loki remote write #4941
• Loki JSON write path: feat: support Loki JSON write #5288
• handmade label parsing in protobuf write path: perf: parse Loki labels in protobuf write path #5305
• update(and self maintain) Loki proto file
• support structured metadata
• support Pipeline process in Loki insertion

TO BE DISCUSSED

• use SKIPPING INDEX instead of PRIMARY KEY upon insertion
• respect existing table's column semantics(tag or field)

Implementation challenges

No response

[evenyag]

• handmade label parsing in protobuf write path

We can postpone this. This may not be the bottleneck of Loki write.