[BE] Expose new metrics available in pg_stat_monitor 0.9

User Impact:
New metrics from pg_stat_monitor 0.9 will be presented on the boards.

How to test:

1. Go to Grafana.
2. Do some queries.
3. Open QAN
4. Select queries for right service
5. Open Chrome Debugger (F12, it should be same for another browsers)
6. Check request /GetMetrics for right query
    
7. It should contains plans_calls, wal_fpi and wal_records property.

Suggested implementation:
1. Modify structure for pg_stat_monitor 0.9 in pmm-agent

2. Modify API

3. Use this in pmm-managed, qan-api2

4.  Modify tests

Original report:
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QAN already collects some data from Postgres extensions pg_stat_statements/pg_stat_monitor like "Shared Blocks Written", "Shared Blocks Read" etc.

Full list of currently gathered/presented columns in QAN and their pg_stat_statement/monitor names:

• Rows Sent (rows)
• Shared Blocks Written (shared_blks_written)
• Shared Blocks Read (shared_blks_read)
• Shared Block Cache Hits (shared_blks_hit)
• Shared Blocks Dirtied (shared_blks_dirtied)
• User CPU time (cpu_user_time *from pg_stat_monitor)
• System CPU time (cpu_sys_time *from pg_stat_monitor)
• Writing Blocks Time (blk_write_time)
• Reading Blocks Time (blk_read_time)

pg_stat_statements and pg_stat_monitor, however, provide many more metrics that could be useful. In particular, in PG 13 the planning time was separated into a separate metric, and WAL write information was added. pg_stat_monitor also has that, but additionally also shows the number of plans for the particular statement.

In general, ideally, we want to see every column of pg_stat_statements/monitor in QAN. With some columns, however, that will require a version-specific code. Perhaps, min/max/mean/stddev time distribution columns are redundant in QAN, though see https://jira.percona.com/browse/PMM-8057 for where that can be useful potentially.