Add transaction.result as a top-level label to transaction duration and span breakdown metrics

## Description of the issue

The APM UI now presents users with [contextual filters](https://www.elastic.co/guide/en/kibana/current/filters.html#contextual-filters), one of which is the transaction result.

Unfortunately, this interacts badly with breakdown metrics, which are not labelled with the transaction result. If you select a transaction result filter, then you end up with something like the following - note that the "Time spent by span type" visualisation is empty:


(Image courtesy of Ron.)

## What we are voting on

Add `transaction.result` as a top-level label to transaction duration and span breakdown metrics.

## Vote

| Agent | Yes | No | Indifferent | N/A | Link to agent issue
| --------|:----:|:---:|:-----------:|:----:|:-------------------:|
| .NET |

• [ ]

|

• [ ]

|

• [ ]

|

• [ ]

|
| Go |

• [x]

|

• [ ]

|

• [ ]

|

• [ ]

|
| Java |

• [ ]

|

• [x]

|

• [ ]

|

• [ ]

|
| Node.js |

• [ ]

|

• [ ]

|

• [ ]

|

• [ ]

|
| Python |

• [ ]

|

• [ ]

|

• [ ]

|

• [ ]

|
| Ruby |

• [ ]

|

• [ ]

|

• [ ]

|

• [ ]

|
| RUM |

• [ ]

|

• [ ]

|

• [ ]

|

• [x]

|