In the '.env' file, a 'DB_PREFIX' sets the prefix that should be used
on every table name. When writing an SQL query the 'DB_PREFIX' value
has to be prefixed to the table name.
The repository PgSqlRepository, MySqlRepository and SqliteRepository,
located in 'app/Repositories/Metric/' did not apply this prefix on
the 'metric_points' table. The problem occured only when we set a
'DB_PREFIX' not null, the rest of the application worked correctly but
the part about 'metric_points' couldn't work, saying the table was
inexistant.
A method was added in the repository AbstractMetricRepository to get
the 'metric_points' table name with the prefix if there is one.
This method is used in the three repositories to get the right table
name.
Note: This problem was present in 2.3, but was already fixed in 2.4 so
there is no need to apply this commit on the 2.4 branch.
See: CachetHQ/Cachet/#2955
