Fix prometheus true/false parsing

1/0 instead of true/false

Replication lag

Added replication lag gauge metrics

1.0.1

Fixed

• Fix case when box.cfg is not called for basic tarantool stats

1.0.0

Added

• Support for counter, gauge and histogram metrics
• Exporting metrics to a prometheus plaintext format
• Serving metrics via tarantool 'http' module
• Collecting basic tarantool stats: memory, request count and tuple counts by space
• Luarock-based packaging
• Basic unit tests