Add mqtt metadata spec and fix sendgrid json sample (#2525)

* add mqtt metadata spec and fix sendgrid json sample

Signed-off-by: yaron2 <schneider.yaron@live.com>

* update phrasing

Signed-off-by: yaron2 <schneider.yaron@live.com>

Co-authored-by: Mark Fussell <markfussell@gmail.com>

daprdocs/content/en/reference/components-reference/supported-bindings/mqtt.md

@@ -136,6 +136,20 @@ This component supports **output binding** with the following operations:
 
 - `create`
 
+## Set topic per-request
+
+You can override the topic in component metadata on a per-request basis:
+
+```json
+{
+  "operation": "create",
+  "metadata": {
+    "topic": "myTopic"
+  },
+  "data": "<h1>Testing Dapr Bindings</h1>This is a test.<br>Bye!"
+}
+```
+
 ## Related links
 
 - [Basic schema for a Dapr component]({{< ref component-schema >}})

daprdocs/content/en/reference/components-reference/supported-bindings/sendgrid.md

@@ -60,6 +60,7 @@ You can specify any of the optional metadata properties on the output binding re
 
 ```json
 {
+  "operation": "create",
   "metadata": {
     "emailTo": "changeme@example.net",
     "subject": "An email from Dapr SendGrid binding"
@@ -67,6 +68,7 @@ You can specify any of the optional metadata properties on the output binding re
   "data": "<h1>Testing Dapr Bindings</h1>This is a test.<br>Bye!"
 }
 ```
+
 ## Related links
 
 - [Basic schema for a Dapr component]({{< ref component-schema >}})