OxQL `QuerySummary` should inclue the actual SQL queries it runs

[bnaecker]

We're storing a summary of the SQL queries run against ClickHouse for each OxQL query, in this type:

omicron/oximeter/db/src/client/query_summary.rs

Line 90 in 68bfc4b

pub struct QuerySummary {

This stores the duration, and number of bytes / rows accessed. We should also include the text of the actual SQL query we're running.

[bnaecker]

cc @jmcarp @lgfa29