Aligning on how errors might inherit information from the current transaction

## Description of the issue

Both errors and transaction have a `context`. They are almost the same and so it (perhaps) makes sense to share data between the two.

There are options thought that will alter how much information each one of them carries. `capture_body`for example can be set to only capture errors bodies.

As @beniwohli noted, things may also very well have changed from when the transaction's context is generated before the transaction and when the error's context is generated when everything explodes.

Do the agents copy over `tags`, `custom` and `user` for example? Does it override and if so in what order?

As discussed briefly in Slack, this is something we might want to align on. Or it might not be. Let's do the check box dance 💃

## What we are voting on

Let's vote on whether or not we think it's worth it to align on this conceptually. Implementations might differ, but at _strategy_ can be shared.

## Vote

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

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|https://github.com/elastic/apm-agent-dotnet/issues/194
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| Python |

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| Ruby |

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| RUM |

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