refactor: decouple config names and values

[taimoorzaeem]

Decouples (or I should say "de-tuple" 🫤 ) the config names and values in config dumping. This opens up space for easier implementation of #4154. Not sure if that is the best way to refactor this. Would love any suggestions.

[steve-chavez]

I think we do need a SSoT for the config names, but the refactor is not looking like a gain in maintenance (has more LOCs).

@taimoorzaeem As a shorter step towards #4154, how about first solving the duplication problem for the sample config in the CLI?

postgrest/src/PostgREST/CLI.hs

Lines 126 to 238 in c1eb8af

	exampleConfigFile :: [Char]
	exampleConfigFile =
	[str|## Admin server used for checks. It's disabled by default unless a port is specified.
	|# admin-server-port = 3001
	|
	|## The database role to use when no client authentication is provided
	|# db-anon-role = "anon"
	|
	|## Notification channel for reloading the schema cache
	|db-channel = "pgrst"
	|
	|## Enable or disable the notification channel
	|db-channel-enabled = true
	|
	|## Enable in-database configuration
	|db-config = true
	|
	|## Function for in-database configuration
	|## db-pre-config = "postgrest.pre_config"
	|
	|## Extra schemas to add to the search_path of every request
	|db-extra-search-path = "public"
	|
	|## Limit rows in response
	|# db-max-rows = 1000
	|
	|## Allow getting the EXPLAIN plan through the `Accept: application/vnd.pgrst.plan` header
	|# db-plan-enabled = false
	|
	|## Number of open connections in the pool
	|db-pool = 10
	|
	|## Time in seconds to wait to acquire a slot from the connection pool
	|# db-pool-acquisition-timeout = 10
	|
	|## Time in seconds after which to recycle pool connections
	|# db-pool-max-lifetime = 1800
	|
	|## Time in seconds after which to recycle unused pool connections
	|# db-pool-max-idletime = 30
	|
	|## Allow automatic database connection retrying
	|# db-pool-automatic-recovery = true
	|
	|## Stored proc to exec immediately after auth
	|# db-pre-request = "stored_proc_name"
	|
	|## Enable or disable prepared statements. disabling is only necessary when behind a connection pooler.
	|## When disabled, statements will be parametrized but won't be prepared.
	|db-prepared-statements = true
	|
	|## The name of which database schema to expose to REST clients
	|db-schemas = "public"
	|
	|## How to terminate database transactions
	|## Possible values are:
	|## commit (default)
	|## Transaction is always committed, this can not be overriden
	|## commit-allow-override
	|## Transaction is committed, but can be overriden with Prefer tx=rollback header
	|## rollback
	|## Transaction is always rolled back, this can not be overriden
	|## rollback-allow-override
	|## Transaction is rolled back, but can be overriden with Prefer tx=commit header
	|db-tx-end = "commit"
	|
	|## The standard connection URI format, documented at
	|## https://www.postgresql.org/docs/current/libpq-connect.html#LIBPQ-CONNSTRING
	|db-uri = "postgresql://"
	|
	|# jwt-aud = "your_audience_claim"
	|
	|## Jspath to the role claim key
	|jwt-role-claim-key = ".role"
	|
	|## Choose a secret, JSON Web Key (or set) to enable JWT auth
	|## (use "@filename" to load from separate file)
	|# jwt-secret = "secret_with_at_least_32_characters"
	|jwt-secret-is-base64 = false
	|
	|## Enables JWT Cache and sets its max size, disables caching with 0
	|# jwt-cache-max-entries = 0
	|
	|## Logging level, the admitted values are: crit, error, warn, info and debug.
	|log-level = "error"
	|
	|## Log the requested SQL query at the current log-level.
	|log-query = "disabled"
	|
	|## Determine if the OpenAPI output should follow or ignore role privileges or be disabled entirely.
	|## Admitted values: follow-privileges, ignore-privileges, disabled
	|openapi-mode = "follow-privileges"
	|
	|## Base url for the OpenAPI output
	|openapi-server-proxy-uri = ""
	|
	|## Configurable CORS origins
	|# server-cors-allowed-origins = ""
	|
	|server-host = "!4"
	|server-port = 3000
	|
	|## Allow getting the request-response timing information through the `Server-Timing` header
	|server-timing-enabled = false
	|
	|## Unix socket location
	|## if specified it takes precedence over server-port
	|# server-unix-socket = "/tmp/pgrst.sock"
	|
	|## Unix socket file mode
	|## When none is provided, 660 is applied by default
	|# server-unix-socket-mode = "660"
	|]

Then the benefit of the refactor will be clear.

[taimoorzaeem]

postgrest/src/PostgREST/CLI.hs

Lines 143 to 151 in f295130

	|## Function for in-database configuration
	|## db-pre-config = "postgrest.pre_config"
	|
	|## Extra schemas to add to the search_path of every request
	|db-extra-search-path = "public"
	|
	|## Limit rows in response
	|# db-max-rows = 1000
	|

The description before each config is understandable but can we afford to lose this trailing ## and # just before in each config here for instance, in db-pre-config and db-max-rows? Not sure what the means TBH.

My idea is to utilize Config.toText and intercalate with the description list. Just these trailing ## and # get in the way.

[steve-chavez]

IIRC # db-max-rows = 1000 has the # because it's actually not set, there's no default. The configs that don't have a # prefix have a default like db-schemas = "public".

So the above means that there's an additional field for a type (that has possible values: default or empty), that would translate to those commented out configs.

I guess we put ## in the description for some tools (like vim) that automatically un-comment by removing one #, so the config remains valid.

It would be just a nicety to keep them as is.

[laurenceisla]

IIRC # db-max-rows = 1000 has the # because it's actually not set, there's no default.

Agree, these are the ones labeled as "Default: n/a" in the docs (or in the case above when: default = not set). Note that this doesn't seem to be consistent right now, cause there are configs that are commented when they have a default value (like the db-pool... examples), so they shouldn't be commented anymore. I think these are specified as Maybe types in the AppConfig, so that could be a way to detect them programmatically.