Update postgres scaler doc for aad (#1437)

* Update postgres scaler doc to include info about Azure access token auth

Signed-off-by: Ferdinand de Baecque <45566171+Ferdinanddb@users.noreply.github.com>

* minor changes to respect conventions

Signed-off-by: Ferdinand de Baecque <45566171+Ferdinanddb@users.noreply.github.com>

---------

Signed-off-by: Ferdinand de Baecque <45566171+Ferdinanddb@users.noreply.github.com>

content/docs/2.15/scalers/postgresql.md

@@ -11,24 +11,27 @@ go_file = "postgresql_scaler"
 
 This specification describes the `postgresql` trigger that scales based on a PostgreSQL query
 
-The PostgreSQL scaler allows for two connection options:
+The PostgreSQL scaler allows for three connection options:
 
 A user can offer a full connection string
-(often in the form of an environment variable secret)
+(often in the form of an environment variable secret).
 
 - `connectionFromEnv` - PostgreSQL connection string that should point to environment variable with valid value.
 
 Alternatively, a user can specify individual
 arguments (host, userName, password, etc.), and the scaler will form a connection string
 internally.
 
-- `host:` - Service URL to postgresql. Note that you should use a full svc URL as KEDA will need to contact postgresql from a different namespace.
-- `userName:` - Username for postgresql user.
+- `host` - Service URL to postgresql. Note that you should use a full svc URL as KEDA will need to contact postgresql from a different namespace.
+- `userName` - Username for postgresql user.
 - `passwordFromEnv` Password for postgresql user.
 - `port` - Postgresql port.
 - `dbName` - Postgresql Database name.
 - `sslmode` - SSL policy for communicating with database.
 
+It is also possible to leverage a `TriggerAuthentication` object having the `azure-workload`'s provider type to connect to an Azure Postgres Flexible Server resource through an UAMI Azure managed identity.  
+More details and an example are provided down below.
+
 Finally, a user inserts a query that returns the desired value.
 
 - `query` - What query to poll postgresql with. Query must return an integer.
@@ -70,7 +73,7 @@ triggers:
 
 ### Authentication Parameters
 
-You can authenticate by using a password or store the password within the connectionString.
+You can authenticate by using a password, or store the password within the connectionString, or leverage Azure Access Token authentication to connect to a Azure Postgres Flexible Server.
 
 **Connection String Authentication:**
 
@@ -85,7 +88,7 @@ You can authenticate by using a password or store the password within the connec
 - `dbName` - PostgreSQL Database name.
 - `sslmode` - SSL policy for communicating with database.
 
-### Example
+#### Example
 
 ```yaml
 apiVersion: keda.sh/v1alpha1
@@ -105,3 +108,65 @@ spec:
         query: "SELECT ceil(COUNT(*)::decimal / 16) FROM task_instance WHERE state='running' OR state='queued';"
         targetQueryValue: 1
 ```
+
+**Azure Access Token authentication:**
+
+#### Prerequisites:
+- The UAMI should be able to access the Azure Postgres Flexible Server, [refer to this link for more info](https://learn.microsoft.com/en-us/azure/postgresql/flexible-server/how-to-connect-with-managed-identity#create-an-azure-database-for-postgresql-flexible-server-user-for-your-managed-identity).
+- The UAMI should be granted access to the table mentioned in the query performed by KEDA.  
+This can be achieved by:
+  - creating a group role to allow access to the particular schema where the table queried by KEDA is persisted, and then assign the newly created Postgres user identity from previous step to this group role.
+  - granting permission on the table queried by KEDA to the newly created Postgres user from previous step, via a query that looks like  
+  `GRANT ALL ON <TABLE_REF> TO "<AZURE_UAMI_NAME>";`.
+
+Next, a user can specify individual arguments (host, userName, password, etc.), and the scaler will form a connection string internally. An access token, which will act as a password, will be retrieved each time KEDA performs its process.
+  - `host` - FQDN of the Azure Postgres Flexible Server.
+  - `userName` - Name of the UAMI Azure identity (`<AZURE_UAMI_NAME>`).
+  - `port` - Postgresql port (the default value is `"5432"`, please have a look at the `Remarks` down below).
+  - `dbName` - Postgresql Database name.
+  - `sslmode` - SSL policy for communicating with database (the value should be `require`).
+
+
+#### Remarks
+
+- While the Azure Postgres Flexible Server resource provides a [`PGBouncer`](https://learn.microsoft.com/en-us/azure/postgresql/flexible-server/concepts-pgbouncer) feature which opens a port `6432` to interact with the server, this access token authentication's feature was not working properly while using the `PGBouncer` port, but it worked without issues while using the default server's port. Therefore, KEDA should use the Postgres server's default port, but the other applications (i.e. Airflow, ...) deployed on the same Kubernetes cluster can use the `PGBouncer` port.
+
+
+#### Example
+
+```yaml
+apiVersion: keda.sh/v1alpha1
+kind: TriggerAuthentication
+metadata:
+  name: azure-pg-flex-auth
+spec:
+  podIdentity:
+    provider: azure-workload
+    # Optional-> identityId: <UAMI_IDENTITY_ID>
+    # Optional-> identityTenantId: <UAMI_TENANT_ID>
+
+---
+
+apiVersion: keda.sh/v1alpha1
+kind: ScaledObject
+metadata:
+  name: airflow-worker
+spec:
+  scaleTargetRef:
+    name: airflow-worker
+  pollingInterval: 10   # Optional. Default: 30 seconds
+  cooldownPeriod: 30    # Optional. Default: 300 seconds
+  maxReplicaCount: 10   # Optional. Default: 100
+  triggers:
+    - type: postgresql
+      authenticationRef:
+        name: azure-pg-flex-auth
+      metadata:
+        host: <AZURE_POSTGRES_FLEX_SERVER_FQDN>
+        port: "5432"
+        userName: <UAMI_NAME>
+        dbName: <DB_NAME>
+        sslmode: require
+        query: "SELECT ceil(COUNT(*)::decimal / 16) FROM task_instance WHERE state='running' OR state='queued';"
+        targetQueryValue: 1
+```