Ability to log a stack trace

[benkehoe]

People like to put their stack traces in their structured logs, so it'd be nice to make that simpler. Something like:

try:
    # do something
except SomeError as e:
    metrics.add_stack_trace("property_key", "optional_value")
    # handle error

and have my log look something like:

{
  "property_key": {
    "value": "optional_value",
    "error_type": "module.SomeError",
    "error_str": "result of str(e)",
    "stack_trace": [
      // traceback.format_stack()
    ]
}

[mwarkentin]

@jaredcnance looks like this could be closed by #41