Breakdown graphs: introduce config option instead of special value for DISABLE_METRICS

## Description of the issue

The current [spec](https://docs.google.com/document/d/1-_LuC9zhmva0VvLgtI0KcHuLzNztPHbcM0ZdlcPUl64/edit#heading=h.12wulrccoi9k) proposes to use a special value (`span.self_time`) for `DISABLE_METRICS` to disable collection of breakdown metrics. This introduces special cases into a mechanism that did not have any previous considerations for special casing, which can be problematic (in the case of the Python agent, it would require a substantial refactor of how `DISABLE_METRICS` currently works).

Instead, I propose to use a boolean config option `BREAKDOWN_METRICS`, with default to `True`. While this sadly adds to the proliferation of config options, I think it is preferable to overloading existing config options with special meaning.

## What we are voting on

Should we add a new config option to disable/enable collection of breakdown metrics?

## Vote

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

• [x]

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| https://github.com/elastic/apm-agent-dotnet/issues/227
| Go |

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• [x]

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| https://github.com/elastic/apm-agent-go/issues/597
| Java |

• [ ]

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• [x]

|

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|https://github.com/elastic/apm-agent-java/pull/743
| Node.js |

• [x]

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| Python |

• [x]

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| Ruby |

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• [x]

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| RUM |

• [ ]

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|

• [x]

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cc @elastic/apm-agent-devs