Implement statistics API

[akudiyar]

For monitoring the internal driver state under load, the users should be able to get the following momentary parameters from the driver API:

• Current number of connections (dead, alive)
• Total number of processed requests
• For each connection:
  • Time when the connection established
  • Total number of processed requests
  • Number of requests currently being processed
  • Last time of any activity within the connection

Requirements:

• Performance of the operations with the driver shouldn't be noticeably affected
• Collecting the statistics via API should not affect the performance (it may be called periodically by the user)
• If the driver is closed, its "current" counters must be equal to zero
• When the driver is created, all counters must be initialized as zero

Proposed API:

New method TarantoolClient#statistics() which returns the following structure:

class TarantoolStatistics {
     public int connectionsTotal;
     public int connectionsAlive;
     public BigDecimal requestsTotal;
     public Map<String, List<TarantoolConnectionStatistics>> connectionsStatistics; // Remote address mapped to connections
}

class TarantoolConnectionStatistics {
     public boolean isAlive;
     public LocalDateTime establishedAt;
     public LocalDateTime closedAt; // will be null if is not closed yet
     public BigDecimal requestsTotal;
     public int requestsCurrent;
     public LocalDateTime lastActivityTime; // may be not very accurate, with the precision of milliseconds
}

[dkasimovskiy]

Looks like an epic.