Bump labkit v1.2.0 and add database metrics

Related to: #238 (closed)

This MR bumps labkit to v1.2.0 and takes advantage of the included database metrics.

Sample output:

HELP go_sql_dbstats_connections_idle The number of idle connections.
# TYPE go_sql_dbstats_connections_idle gauge
go_sql_dbstats_connections_idle{db_name="registry_test"} 0
# HELP go_sql_dbstats_connections_in_use The number of connections currently in use.
# TYPE go_sql_dbstats_connections_in_use gauge
go_sql_dbstats_connections_in_use{db_name="registry_test"} 1
# HELP go_sql_dbstats_connections_max_idle_closed_count_total The total number of connections closed due to SetMaxIdleConns.
# TYPE go_sql_dbstats_connections_max_idle_closed_count_total counter
go_sql_dbstats_connections_max_idle_closed_count_total{db_name="registry_test"} 1165
# HELP go_sql_dbstats_connections_max_lifetime_closed_count_total The total number of connections closed due to SetConnMaxLifetime.
# TYPE go_sql_dbstats_connections_max_lifetime_closed_count_total counter
go_sql_dbstats_connections_max_lifetime_closed_count_total{db_name="registry_test"} 0
# HELP go_sql_dbstats_connections_max_open The limit of open connections to the database.
# TYPE go_sql_dbstats_connections_max_open gauge
go_sql_dbstats_connections_max_open{db_name="registry_test"} 0
# HELP go_sql_dbstats_connections_open The number of established connections both in use and idle.
# TYPE go_sql_dbstats_connections_open gauge
go_sql_dbstats_connections_open{db_name="registry_test"} 1
# HELP go_sql_dbstats_connections_wait_seconds_total The total time blocked waiting for a new connection.
# TYPE go_sql_dbstats_connections_wait_seconds_total counter
go_sql_dbstats_connections_wait_seconds_total{db_name="registry_test"} 0
# HELP go_sql_dbstats_connections_waits_total The total number of connections waited for.
# TYPE go_sql_dbstats_connections_waits_total counter
go_sql_dbstats_connections_waits_total{db_name="registry_test"} 0