WL#12600: InnoDB: Add metrics for undo truncation

Add metrics to information_schema.innodb_metrics for the background activities
during undo tablespace truncation. These metrics should be available with this
command;

select * from information_schema.innodb_metrics where subsystem = 'undo';

Two new metrics will be added to the "purge" subsystem to count and time the
effort to clean up old undo logs, and eight new metrics are added to a new
subsystem called "undo".

These are the metrics being added:

    {"purge_truncate_history_count", "purge",
     "Number of times the purge thread attempted to truncate undo history",
     MONITOR_NONE, MONITOR_DEFAULT_START, MONITOR_PURGE_TRUNCATE_HISTORY_COUNT},

    {"purge_truncate_history_usec", "purge",
     "Time (in microseconds) the purge thread spent truncating undo history.",
     MONITOR_NONE, MONITOR_DEFAULT_START,
     MONITOR_PURGE_TRUNCATE_HISTORY_MICROSECOND},

    /* ========== Counters for Undo Tablespace Truncation ========== */
    {"module_undo", "undo", "Undo Truncation", MONITOR_MODULE,
     MONITOR_DEFAULT_START, MONITOR_UNDO_TRUNCATE},


    {"undo_truncate_count", "undo",
     "Number of times undo truncation was initiated", MONITOR_NONE,
     MONITOR_DEFAULT_START, MONITOR_UNDO_TRUNCATE_COUNT},

    {"undo_truncate_sweep_count", "undo",
     "Number of times undo truncation invalidates old pages from the buffer "
     "pool",
     MONITOR_NONE, MONITOR_DEFAULT_START, MONITOR_UNDO_TRUNCATE_SWEEP_COUNT},

    {"undo_truncate_sweep_usec", "undo",
     "Time (in microseconds) spent during undo truncation invalidating old "
     "pages from the buffer pool",
     MONITOR_NONE, MONITOR_DEFAULT_START,
     MONITOR_UNDO_TRUNCATE_SWEEP_MICROSECOND},

    {"undo_truncate_start_logging_count", "undo",
     "Number of times during undo truncation a log file was started",
     MONITOR_NONE, MONITOR_DEFAULT_START,
     MONITOR_UNDO_TRUNCATE_START_LOGGING_COUNT},

    {"undo_truncate_flush_count", "undo",
     "Number of times undo truncation flushed new pages from the buffer pool "
     "to disk",
     MONITOR_NONE, MONITOR_DEFAULT_START, MONITOR_UNDO_TRUNCATE_FLUSH_COUNT},

    {"undo_truncate_flush_usec", "undo",
     "Time (in microseconds) spent during undo truncation flushing new pages "
     "from the buffer pool to disk",
     MONITOR_NONE, MONITOR_DEFAULT_START,
     MONITOR_UNDO_TRUNCATE_FLUSH_MICROSECOND},

    {"undo_truncate_done_logging_count", "undo",
     "Number of times during undo truncation a log file was deleted",
     MONITOR_NONE, MONITOR_DEFAULT_START,
     MONITOR_UNDO_TRUNCATE_DONE_LOGGING_COUNT},

    {"undo_truncate_usec", "undo",
     "Time (in microseconds) spent to process undo truncation", MONITOR_NONE,
     MONITOR_DEFAULT_START, MONITOR_UNDO_TRUNCATE_MICROSECOND},