add mongodb log type for kubernetes and file input sources

[jsirianni]

When using mongodb on kubernetes, the wrong log type label is applied. This is because it was only being set at the file input operator. I have removed this, and added an add operator to the end of the pipeline. This way, the log type is added regardless of the log source (kubernetes or file input).

before: "log_type": "k8s.container",

{
  "timestamp": "2021-08-11T14:50:57.323Z",
  "severity": 30,
  "severity_text": "I",
  "labels": {
    "k8s-ns/kubernetes.io/metadata.name": "default",
    "k8s-pod/app": "mongo",
    "k8s-pod/pod-template-hash": "55457b675",
    "log_type": "k8s.container",
    "plugin_id": "kubernetes_input",
    "stream": "stdout"
  },
  "resource": {
    "k8s.cluster.name": "stanza_example",
    "k8s.container.id": "c338a244244005d88db8e434b762c939b94102cab7babba57980b4f247b5aae1",
    "k8s.container.name": "wordpress",
    "k8s.deployment.name": "mongodb",
    "k8s.namespace.name": "default",
    "k8s.namespace.uid": "f0d2a872-ea9c-4ca6-a92e-b31571bc76d7",
    "k8s.node.name": "",
    "k8s.pod.name": "mongodb-55457b675-79t5q",
    "k8s.pod.uid": "3ca4719b-7368-4fd8-93db-d625429719ea",
    "k8s.replicaset.name": "mongodb-55457b675"
  },
  "record": {
    "attr": {},
    "component": "STORAGE",
    "context": "Checkpointer",
    "id": 22430,
    "message": "[1628693457:323919][1:0x7f756da2f700], WT_SESSION.checkpoint: [WT_VERB_CHECKPOINT_PROGRESS] saving checkpoint snapshot min: 11, snapshot max: 11 snapshot count: 0, oldest timestamp: (0, 0) , meta checkpoint timestamp: (0, 0) base write gen: 7",
    "message_type": "WiredTiger message"
  }
}

after: "log_type": "mongodb",

{
  "timestamp": "2021-08-11T16:10:35.778Z",
  "severity": 30,
  "severity_text": "I",
  "labels": {
    "k8s-ns/kubernetes.io/metadata.name": "default",
    "k8s-pod/app": "mongo",
    "k8s-pod/pod-template-hash": "55457b675",
    "log_type": "mongodb",
    "plugin_id": "kubernetes_input",
    "stream": "stdout"
  },
  "resource": {
    "k8s.cluster.name": "stanza_example",
    "k8s.container.id": "2847c455791fa59a95eb8421dfa4e0205ec540196148a88aed3391b3159654e9",
    "k8s.container.name": "wordpress",
    "k8s.deployment.name": "mongodb",
    "k8s.namespace.name": "default",
    "k8s.namespace.uid": "f0d2a872-ea9c-4ca6-a92e-b31571bc76d7",
    "k8s.node.name": "",
    "k8s.pod.name": "mongodb-55457b675-hkqvj",
    "k8s.pod.uid": "ce262204-49b1-4360-b159-729b6f2334cb",
    "k8s.replicaset.name": "mongodb-55457b675"
  },
  "record": {
    "attr": {},
    "component": "STORAGE",
    "context": "Checkpointer",
    "id": 22430,
    "message": "[1628698235:778708][1:0x7f66684e9700], WT_SESSION.checkpoint: [WT_VERB_CHECKPOINT_PROGRESS] saving checkpoint snapshot min: 5, snapshot max: 5 snapshot count: 0, oldest timestamp: (0, 0) , meta checkpoint timestamp: (0, 0) base write gen: 7",
    "message_type": "WiredTiger message"
  }
}

[BinaryFissionGames]

Looks good!