Document the new logs endpoint and env variable

**What**
- Document the new logs handler url env variable in the gateway readme
- Document the logs handler in the swagger docs
Signed-off-by: Lucas Roesler <roesler.lucas@gmail.com>

api-docs/swagger.yml

@@ -332,6 +332,41 @@ paths:
           description: Not Found
         '500':
           description: Internal Server Error
+  '/system/logs':
+      get:
+        summary: Get a stream of the logs for a specific function
+        produces:
+          - application/x-ndjson
+      parameters:
+      - in: query
+        name: name
+        description: Function name
+        type: string
+        required: true
+      - in: query
+        name: since
+        description: Only return logs after a specific date (RFC3339)
+        type: string
+        required: false
+      - in: query
+        name: tail
+        description: Sets the maximum number of log messages to return, <=0 means unlimited
+        type: integer
+        required: false
+      - in: query
+        name: follow
+        description: When true, the request will stream logs until the request timeout
+        type: boolean
+        required: false
+      responses:
+        '200':
+          description: Newline delimited stream of log messages
+          schema:
+            $ref: '#/definitions/LogEntry'
+        '404':
+          description: Not Found
+        '500':
+          description: Internal Server Error
   '/system/info':
     get:
       summary: Get info such as provider version number and provider orchestrator
@@ -559,6 +594,22 @@ definitions:
         example: changeme
     required:
     - name
+  LogEntry:
+    type: object
+    property:
+      name:
+        type: string
+        description: the function name
+      instance:
+        type: string
+        description: the name/id of the specific function instance
+      timestamp:
+        type: string
+        format: date-time
+        description: the timestamp of when the log message was recorded
+      text:
+        type: string
+        description: raw log message content
   SecretName:
     type: object
     properties:

gateway/README.md

 # Gateway
 
-The API Gateway provides an external route into your functions and collects Cloud Native metrics through Prometheus. The gateway also has a UI built-in which can be used to deploy your own functions or functions from the OpenFaaS Function Store then invoke them. 
+The API Gateway provides an external route into your functions and collects Cloud Native metrics through Prometheus. The gateway also has a UI built-in which can be used to deploy your own functions or functions from the OpenFaaS Function Store then invoke them.
 
 The gateway will scale functions according to demand by altering the service replica count in the Docker Swarm or Kubernetes API. Custom alerts generated by AlertManager are received on the /system/alert endpoint.
 
@@ -45,7 +45,9 @@ See the [openfaas/store](https://github.com/openfaas/store) repo for more.
 
 ## Logs
 
-Logs are available at the function level and can be accessed through Swarm or Kubernetes using native tooling. You can also install a Docker logging driver to aggregate your logs. By default functions will not write the request and response bodies to stdout. You can toggle this behaviour by setting `read_debug` for the request and `write_debug` for the response.
+Logs are available at the function level via the API.
+
+You can also install a Docker logging driver to aggregate your logs. By default functions will not write the request and response bodies to stdout. You can toggle this behaviour by setting `read_debug` for the request and `write_debug` for the response.
 
 ## Tracing
 
@@ -60,13 +62,14 @@ X-Call-Id
 Within a function this is available as `Http_X_Call_Id`.
 
 ## Environmental overrides
-The gateway can be configured through the following environment variables: 
+The gateway can be configured through the following environment variables:
 
 | Option                 | Usage             |
 |------------------------|--------------|
 | `write_timeout`        | HTTP timeout for writing a response body from your function (in seconds). Default: `8`  |
 | `read_timeout`         | HTTP timeout for reading the payload from the client caller (in seconds). Default: `8` |
 | `functions_provider_url`             | URL of upstream [functions provider](https://github.com/openfaas/faas-provider/) - i.e. Swarm, Kubernetes, Nomad etc  |
+| `logs_provider_url` | URL of the upstream function logs api provider, optional, when empty the `functions_provider_url` is used |
 | `faas_nats_address`          | Address of NATS service. Required for asynchronous mode |
 | `faas_nats_port`    | Port for NATS service. Requrired for asynchronous mode |
 | `faas_prometheus_host`         | Host to connect to Prometheus. Default: `"prometheus"` |