add mongodb scaler doc (#343)

Signed-off-by: 高强 <wb-gq555900@alibaba-inc.com>

Co-authored-by: 高强 <wb-gq555900@alibaba-inc.com>

content/docs/2.1/scalers/mongodb.md

@@ -0,0 +1,116 @@
++++
+title = "MongoDB"
+layout = "scaler"
+maintainer = "Community"
+description = "Scale applications based on MongoDB queries."
+availability = "v2.1+"
+go_file = "mongodb_scaler"
++++
+
+### Trigger Specification
+
+This specification describes the `mongodb` trigger that scales based on result of MongoDB query.
+
+```yaml
+triggers:
+  - type: mongodb
+    metadata:
+      # Required: database name
+      dbName: "test"
+      # Required: collection name
+      collection: "test_collection"
+      # Required: query expr, used by filter data
+      query: '{"region":"eu-1","state":"running","plan":"planA"}'
+      # Required: according to the number of query result, to scale job
+      queryValue: "1"
+      # Optional: This value will be assign to the metric
+      metricName: "global-metric"
+```
+
+**Parameter list:**
+
+- `dbName`  - Name of the database
+- `collection` - Name of the collection
+- `query` - A MongoDB query that should return single numeric value
+- `queryValue` - A threshold that will define when scaling should occur
+- `metricName` - An optional name to assign to the metric.
+
+To provide information about how to connect to MongoDB you can provide
+
+- `connectionStringFromEnv` - A MongoDB connection string that should point to environment variable with valid value
+
+Or provide more detailed information:
+
+- `host` - The host of the MongoDB server
+- `port` - The port of the MongoDB server
+- `username` - Username to authenticate with to MongoDB database
+- `passwordFromEnv` - Password for the given user
+
+### Authentication Parameters 
+
+You can authenticate by using connection string or password authentication.
+
+**Connection String Authentication:**
+
+- `connectionString`  - Connection string for MongoDB database
+
+**Password Authentication:**
+
+- `password` - Password for the configured user to login to MongoDB database variables.
+
+### Example
+
+Here is an example of how to deploy a scaled Job with the `MongoDB` scale trigger which uses `TriggerAuthentication`.
+
+```yaml
+apiVersion: keda.sh/v1alpha1
+kind: ScaledJob
+metadata:
+  name: mongodb-job
+spec:
+  jobTargetRef:
+    template:
+      spec:
+        containers:
+          - name: mongodb-update
+            image: 1314520999/mongodb-update:latest
+            args:
+            - --connectStr=mongodb://test_user:test_password@mongoDB-svc.mongoDB.svc.cluster.local:27017/test
+            - --dataBase=test
+            - --collection=test_collection
+            imagePullPolicy: IfNotPresent
+        restartPolicy: Never
+    backoffLimit: 1
+  pollingInterval: 30             # Optional. Default: 30 seconds
+  maxReplicaCount: 30             # Optional. Default: 100
+  successfulJobsHistoryLimit: 0   # Optional. Default: 100. How many completed jobs should be kept.
+  failedJobsHistoryLimit: 10      # Optional. Default: 100. How many failed jobs should be kept.
+  triggers:
+    - type: mongodb
+      metadata:
+        dbName: "test"
+        collection: "test_collection"
+        query: '{"region":"eu-1","state":"running","plan":"planA"}'
+        queryValue: "1"
+        metricName: "global-metric"
+      authenticationRef:
+        name: mongodb-trigger
+---
+apiVersion: keda.sh/v1alpha1
+kind: TriggerAuthentication
+metadata:
+  name: mongodb-trigger
+spec:
+  secretTargetRef:
+    - parameter: connectionString
+      name: mongodb-secret
+      key: connect
+---
+apiVersion: v1
+kind: Secret
+metadata:
+  name: mongodb-secret
+type: Opaque
+data:
+  connect: bW9uZ29kYjovL3Rlc3RfdXNlcjp0ZXN0X3Bhc3N3b3JkQG1vbmdvZGItc3ZjLm1vbmdvREIuc3ZjLmNsdXN0ZXIubG9jYWw6MjcwMTcvdGVzdA==
+```