MVCC: memory monitoring

[unera]

1. Tarantool is a high RPS application. So, typical tarantool application will have too many RPS.
2. MVCC: A transaction uses additional memory since the first conflict and until it is closed.

So if You reflect on the pt 1 and 2, You'll find that there can be a situation:

• someone's opened a transaction and hasn't closed it for a long time
• high RPS level brings tarantool to OOM killer

So it would be nice to monitor how much memory MVCC uses.

Min/Max/AVG per txn/total

[unera]

1. All these counters are in box.stat.memtx.mvcc, so don't use memtx, mvcc as keyname parts.
2. Don't use phrases with spaces as key names (tuple stories -> tuples, etc)

[drewdzzz]

Method to get statistics: box.stat.memtx.tx()

  txn:
    statements:
      max: 0
      avg: 0
      total: 0
    user:
      max: 0
      avg: 0
      total: 0
    system:
      max: 0
      avg: 0
      total: 0
  mvcc:
    trackers:
      max: 0
      avg: 0
      total: 0
    conflicts:
      max: 0
      avg: 0
      total: 0
    tuples:
      tracking:
        stories:
          count: 0
          total: 0
        retained:
          count: 0
          total: 0
      used:
        stories:
          count: 0
          total: 0
        retained:
          count: 0
          total: 0
      read_view:
        stories:
          count: 0
          total: 0
        retained:
          count: 0
          total: 0

New format of statistics, @unera, what do you think?

[unera]

ok