Signify that a stack trace was truncated

## Description of the issue

A few agents (Java, Ruby at least) have the option to truncate stack traces to _x_ lines. Currently this is done by simply chopping off the bottom.

However there's no indication in the UI to the user that this has happened, perhaps leading to confusion.

Suggestions for implementation:

- `stacktrace: { truncated: true }`. A simple boolean is likely the simplest solution.
- `stacktrace: { truncated: 9 }`. However a count is probably just as easy to save while giving a bit more information.
- `stacktrace: { truncated: [[0, 3], [6, 10]] }`. If we want to get even fancier in the future, for example doing middle truncation or truncation depending on lines being library or application code, we need to store the exact indexes so the UI can show _...truncated..._ in the right places.

## UPDATE

**We go with the simplest implementation, `stacktrace: { truncated: true }`.**

I suggest we make it optional on the server side for compatibility with older agents but default to `false` in agents when implemented?

## What we are voting on

Whether this is a feasible (and _good enough_) solution. Added a row for APM Server as this is going into the api too.

## Vote

| Agent | Yes | No | Indifferent | N/A | Link to agent issue
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| APM Server |

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| .NET |

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| Go |

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| Java |

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| Node.js |

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| Python |

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| Ruby |

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| RUM |

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