Expose database metrics

Related to: #238 (closed)

This MR exposes database metrics based sql.DBStats to prometheus.

Sample output:

# HELP go_sql_stats_connections_blocked_seconds The total time blocked waiting for a new connection.
# TYPE go_sql_stats_connections_blocked_seconds counter
go_sql_stats_connections_blocked_seconds{db_name="registry_test"} 0
# HELP go_sql_stats_connections_closed_max_idle The total number of connections closed due to SetMaxIdleConns.
# TYPE go_sql_stats_connections_closed_max_idle counter
go_sql_stats_connections_closed_max_idle{db_name="registry_test"} 3216
# HELP go_sql_stats_connections_closed_max_lifetime The total number of connections closed due to SetConnMaxLifetime.
# TYPE go_sql_stats_connections_closed_max_lifetime counter
go_sql_stats_connections_closed_max_lifetime{db_name="registry_test"} 0
# HELP go_sql_stats_connections_idle The number of idle connections.
# TYPE go_sql_stats_connections_idle gauge
go_sql_stats_connections_idle{db_name="registry_test"} 0
# HELP go_sql_stats_connections_in_use The number of connections currently in use.
# TYPE go_sql_stats_connections_in_use gauge
go_sql_stats_connections_in_use{db_name="registry_test"} 1
# HELP go_sql_stats_connections_max_open Maximum number of open connections to the database.
# TYPE go_sql_stats_connections_max_open gauge
go_sql_stats_connections_max_open{db_name="registry_test"} 0
# HELP go_sql_stats_connections_open The number of established connections both in use and idle.
# TYPE go_sql_stats_connections_open gauge
go_sql_stats_connections_open{db_name="registry_test"} 1
# HELP go_sql_stats_connections_waited_for The total number of connections waited for.
# TYPE go_sql_stats_connections_waited_for counter
go_sql_stats_connections_waited_for{db_name="registry_test"} 0