Query Metadata Aggregator [LUCENE-8919]

It would be good if there was a mechanism to allow aggregation of metadata for queries (eg, number of clauses, types of clauses, terms involved etc). This is particularly useful for complex queries with multiple levels of nesting and a high degree of branching. This should help debug query performance issues and draw patterns in case a query is misbehaving. With the QueryVisitor being present, this should be doable.

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Migrated from [LUCENE-8919](https://issues.apache.org/jira/browse/LUCENE-8919) by Atri Sharma (@atris)

Start by reading the existing QueryVisitor API and the query traversal it supports. Define how query metadata such as clause counts, clause types, and involved terms should be aggregated, including nested and branching queries; done means a usable mechanism exists to inspect those patterns for performance debugging.

Written by the indexing model from the issue text.