The idea here is that we collect metrics during Table scans so that we
can better understand what exactly happens during a Table scan.

This is just an initial implementation that gathers/records a few metrics. The idea is to get early feedback on this approach.

Overview of proposed changes

• a ScanReport that contains all relevant metrics from a Scan
• a ScanReporter interface that can be used to collect and report different metrics during a Table scant. Engines can implement this API to customize how a ScanReport is being reported
• a LoggingScanReporter is introduced that logs a ScanReport to the log file. We can think about defaulting to that one or using a no-op scan reporter
• a new Catalog Property scan-reporter-impl is introduced, allowing a Catalog to specify which ScanReporter to use
• a ScanMetrics that carries all relevant metrics during a Table scan. This class should be seen as something that is only used during a scan. At the end of a scan we'll want to create a ScanReport based on the collected metrics
• a Timer and a DefaultTimer were introduced so that we can measure spent time for a table scan
• some other changes, such as DefaultScanMetrics that use native Java Integer/Long counters and a DefaultTimer