refactor: get_stats merge and simplify

[de-sh]

Fixes #XXXX.

Description

• Use serde and Query to abstract away param extraction
• Merge get_stats from logstream.rs(used in both ingestor and standalone) and query_logstream.rs(used in querier)
• Organize imports in affected files

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This PR has:

• been tested to ensure log ingestion and log query works.
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added documentation for new or modified features or behaviors.