Adds all bool/integer/real runtime variables that can be retrieved ths way.
Use the `pg_settings` view to retrieve runtime variables:
https://www.postgresql.org/docs/current/static/view-pg-settings.html
Replaces the use of `SHOW` to retrieve runtime variables.
Only runtime variables with a `vartype` of `bool`, `real`, or `integer`
are currently supported. Uses the `short_desc` field as a description.
This commit deprecates the following metric names:
```
pg_runtime_variable_max_connections
pg_runtime_variable_max_files_per_process
pg_runtime_variable_max_function_args
pg_runtime_variable_max_identifier_length
pg_runtime_variable_max_index_keys
pg_runtime_variable_max_locks_per_transaction
pg_runtime_variable_max_pred_locks_per_transaction
pg_runtime_variable_max_prepared_transactions
pg_runtime_variable_max_standby_archive_delay_milliseconds
pg_runtime_variable_max_standby_streaming_delay_milliseconds
pg_runtime_variable_max_wal_senders
```
They are replaced by equivalent names under `pg_settings` with the exception of
```
pg_runtime_variable_max_standby_archive_delay_milliseconds
pg_runtime_variable_max_standby_streaming_delay_milliseconds
```
which are replaced with
```
pg_settings_max_standby_archive_delay_seconds
pg_settings_max_standby_streaming_delay_seconds
```
Adds approximately 195 new metrics, when considered across all supported
PostgreSQL versions.
Will Rouesnel
Adam Sunderland
Matt Bostock