Add ability to associate an ID with tasks (#27764)
Adds support for capturing the X-Opaque-Id header from a REST request and storing it's value in the tasks that this request started. It works for all user-initiated tasks (not only search).
Closes #23250
Usage:
```
$ curl -H "X-Opaque-Id: imotov" -H "foo:bar" "localhost:9200/_tasks?pretty&group_by=parents"
{
"tasks" : {
"7qrTVbiDQKiZfubUP7DPkg:6998" : {
"node" : "7qrTVbiDQKiZfubUP7DPkg",
"id" : 6998,
"type" : "transport",
"action" : "cluster:monitor/tasks/lists",
"start_time_in_millis" : 1513029940042,
"running_time_in_nanos" : 266794,
"cancellable" : false,
"headers" : {
"X-Opaque-Id" : "imotov"
},
"children" : [
{
"node" : "V-PuCjPhRp2ryuEsNw6V1g",
"id" : 6088,
"type" : "netty",
"action" : "cluster:monitor/tasks/lists[n]",
"start_time_in_millis" : 1513029940043,
"running_time_in_nanos" : 67785,
"cancellable" : false,
"parent_task_id" : "7qrTVbiDQKiZfubUP7DPkg:6998",
"headers" : {
"X-Opaque-Id" : "imotov"
}
},
{
"node" : "7qrTVbiDQKiZfubUP7DPkg",
"id" : 6999,
"type" : "direct",
"action" : "cluster:monitor/tasks/lists[n]",
"start_time_in_millis" : 1513029940043,
"running_time_in_nanos" : 98754,
"cancellable" : false,
"parent_task_id" : "7qrTVbiDQKiZfubUP7DPkg:6998",
"headers" : {
"X-Opaque-Id" : "imotov"
}
}
]
}
}
}
```
c75ac319a6