Making it easier for users to retrieve and submit query plans for ill-performing queries

[jarohen]

One of the first questions we often ask users with query performance issues is to get us the debug logging for crux.query, which contains the attribute stats and join order - let's see if we can make it easier for users to get hold of.

Some ideas:

• We could make this a semi-internal API, no guarantees about the return format
• Setting the :explain? flag on the query dumps the query, relevant attribute stats and join order to stderr?
• Returning explain plans as metadata on query results?
• We could obfuscate the attribute names (and anything else sensitive in the query?), maybe a separate flag?

[refset]

Noting that users can currently call xtdb.query/query-plan-for directly https://github.com/xtdb/xtdb/blob/1454e1ffed7511a1e701e86d373e1fea8a748214/core/src/xtdb/query.clj#L1822