logs: allow disabling log collection in jobspec (#16962)

Some Nomad users ship application logs out-of-band via syslog. For these users
having `logmon` (and `docker_logger`) running is unnecessary overhead. Allow
disabling the logmon and pointing the task's stdout/stderr to /dev/null.

This changeset is the first of several incremental improvements to log
collection short of full-on logging plugins. The next step will likely be to
extend the internal-only task driver configuration so that cluster
administrators can turn off log collection for the entire driver.

---

Fixes: #11175

Co-authored-by: Thomas Weber <towe75@googlemail.com>

website/content/api-docs/client.mdx

@@ -603,7 +603,8 @@ fields:
 
 ## Stream Logs
 
-This endpoint streams a task's stderr/stdout logs.
+This endpoint streams a task's stderr/stdout logs. Note that if logging is set
+to [enabled=false][] for the task, this endpoint will return a 404 error.
 
 | Method | Path                           | Produces     |
 | ------ | ------------------------------ | ------------ |
@@ -832,3 +833,4 @@ $ nomad operator api /v1/client/gc
 ```
 
 [api-node-read]: /nomad/api-docs/nodes
+[enabled=false]: /nomad/docs/job-specification/logs#enabled

website/content/api-docs/jobs.mdx

@@ -491,6 +491,7 @@ $ curl \
           },
           "KillTimeout": 5000000000,
           "LogConfig": {
+            "Enabled": true,
             "MaxFiles": 10,
             "MaxFileSizeMB": 10
           },
@@ -762,6 +763,7 @@ $ curl \
               "Leader": false,
               "Lifecycle": null,
               "LogConfig": {
+                "Enabled": true,
                 "MaxFileSizeMB": 10,
                 "MaxFiles": 10
               },
@@ -979,6 +981,7 @@ $ curl \
               "Leader": false,
               "Lifecycle": null,
               "LogConfig": {
+                "Enabled": true,
                 "MaxFileSizeMB": 10,
                 "MaxFiles": 10
               },
@@ -1145,6 +1148,7 @@ $ curl \
               "Leader": false,
               "Lifecycle": null,
               "LogConfig": {
+                "Enabled": true,
                 "MaxFileSizeMB": 10,
                 "MaxFiles": 10
               },

website/content/api-docs/json-jobs.mdx

@@ -927,6 +927,8 @@ The `Affinity` object supports the following keys:
 The `LogConfig` object configures the log rotation policy for a task's `stdout` and
 `stderr`. The `LogConfig` object supports the following attributes:
 
+- `Enabled` - Enables log collection.
+
 - `MaxFiles` - The maximum number of rotated files Nomad will retain for
   `stdout` and `stderr`, each tracked individually.
 
@@ -940,6 +942,7 @@ a validation error when a job is submitted.
 ```json
 {
   "LogConfig": {
+    "Enabled": true,
     "MaxFiles": 3,
     "MaxFileSizeMB": 10
   }