Added system.stack_trace table.

[alexey-milovidov]

Changelog category (leave one):

• New Feature

Changelog entry (up to few sentences, required except for Non-significant/Documentation categories):
Added system.stack_trace table that allows to look at stack traces of all server threads. This is useful for developers to introspect server state. This fixes #7576

Detailed description (optional):
Usage example:

SET allow_introspection_functions = 1;

WITH arrayMap(x -> demangle(addressToSymbol(x)), trace) AS all SELECT query_id, thread_number, arrayStringConcat(all, '\n') AS res FROM system.stack_trace \G

WITH arrayMap(x -> addressToLine(x), trace) AS all, arrayFilter(x -> x LIKE '%/dbms/%', all) AS dbms SELECT query_id, thread_number, arrayStringConcat(notEmpty(dbms) ? dbms : all, '\n') AS res FROM system.stack_trace \G

[alexey-milovidov]

It looks like RT-signals are not delivered inside Sandbox (Yandex internal job running system that is used in our CI).