Span generator tool

spangen is a cli tool that can generate spans from the command line.

> ./spangen  -id 100 -trace-id 200 -start-micros 1234 -duration-micros 5 -string-tag test:1234 -int-tag count:123 -dataset test_dataset
{
    "id": "MTAw",
    "trace_id": "MjAw",
    "timestamp": 1234,
    "duration": 5,
    "tags": [
        {
            "key": "test",
            "v_str": "1234"
        },
        {
            "key": "count",
            "v_type": 2,
            "v_int64": 123
        },
        {
            "key": "__dataset",
            "v_str": "test_dataset"
        }
    ]
}

Sample run:

> ./spangen --help
Usage of ./spangen:
  -bool-tag value
    	tag formatted as key:value, where value is true or false (repeatable)
  -dataset string
    	set a dataset tag
  -duration-micros int
    	duration of event in microseconds (required)
  -float-tag value
    	tag formatted as key:value, where value is a float (repeatable)
  -id string
    	span id (required unless using -one-off)
  -int-tag value
    	tag formatted as key:value, where value is an integer (repeatable)
  -name string
    	name for the event
  -one-off
    	use a random span id and trace id
  -parent-id string
    	parent id (leave empty for root span)
  -reporter string
    	tell the generator how to report spans; allowed reporters: noop console (default "console")
  -start-micros int
    	start of event in microseconds since epoch (required)
  -string-tag value
    	tag formatted as key:value, where value is a string (repeatable)
  -trace-id string
    	trace id (required unless using -one-off)
  -verbose
    	print results as json object
  -version
    	print version and quit