better P_S instrumentation for PS

P_S (performance_schema) instrumentation for PS (prepared statements) can be improved. This is important for monitoring.

In particular, quoting developers mailing list:

In this regard, there are the following problems with analytics for prepared statements compared to regular queries:
1) Most applications work with prepared statements using the following method when processing a single query - creating a prepared statement, executing the prepared statement, and then dropping the prepared statement. This leads to the fact that query statistics exist for a very short period of time, during which it may not be possible to fetch them.
2) The second problem of prepared statement analytics in comparison with regular queries is the absence of the query hash (digest) and information about the schema_name in which the prepared statement is executed, which makes it difficult to identify unique queries, since a separate record in the Prepared_Statements_instances table is created for each creation of a prepared statement.
3) Execution of prepared statements is possible in two protocols: Binary and text. Execution of a prepared statement in the binary protocol does not create an entry in the events_statements_* tables, but execution in the text protocol creates an entry in the specified tables, which leads to different behaviors of the same prepared statement but in different protocols.
4) For prepared statements, there is no option in getting an example request without a ? placeholder. The SQL_TEXT field in Prepared_Statements_instances contains The prepared statement text, with ? placeholder markers, and in some cases (when executing in the text protocol) it is possible to find only the execute command of this statement, which further requires mapping ? placeholder markers with parameters from the execute request. But there are limitations that the binary protocol does not store EXECUTE requests for prepared statements, and most of the interactions with the database occur through it, through drivers for different programming languages, and another limitation may be the use of variables in the EXECUTE command, the value of which still needs to be obtained.