How should we represent in-flight spans in the UI

[codefromthecrypt]

Currently, an in-flight span shows up as NaN (as it doesn't have a duration). Is this ok? how could we make it look better without misleading people into thinking an in-flight span is actually complete.

$ curl -s localhost:9411/api/v1/spans -H'Content-type: application/json' -d '[
  {
    "traceId": "77047bf9cf04ed5d",
    "id": "77047bf9cf04ed5d",
    "name": "get",
    "timestamp": 1484722951945904,
    "duration": 156470,
    "annotations": [
      {
        "timestamp": 1484722951945904,
        "value": "sr",
        "endpoint": {
          "serviceName": "client"
        }
      },
      {
        "timestamp": 1484722952102374,
        "value": "ss",
        "endpoint": {
          "serviceName": "client"
        }
      }
    ]
  },
  {
    "traceId": "77047bf9cf04ed5d",
    "id": "40ef4e93ab3b1507",
    "name": "get",
    "parentId": "77047bf9cf04ed5d",
    "annotations": [
      {
        "timestamp": 1484722952100852,
        "value": "cs",
        "endpoint": {
          "serviceName": "client"
        }
      }
    ]
  }
]'

[codefromthecrypt]

cc @dragontree101 @cburroughs

[jorgheymans]

In Lens, in-flight spans are shown correctly (no Nan) but there is no visual indication either that it's in-flight. A subtle hint or mouse-over that the request is in-flight is probably still useful.