enhancement: Add events for starting, stopping, and reloading

[ktff]

Closes #3448.

Added events emit:

• Vector is starting.
• Vector is reloading.
• Vector is stopping.

logs respectively. This differs from what it used to be, but it reflects the metrics and is more streamlined.

Open questions

• logs are emitted under fixed target = "vector" to avoid vector::internal_events::process: being logged as the call site. Haven't seen what's our stance on that. It should be possible to extend emit! macro to set target as the actual emit site, or is this the intended behavior? (Edit: it should be fine to use target for now)

• Should we use lowercase vector ? (Edit: use Vector for emphasis)

[lukesteensen]

Looking at this again and taking @jamtur01's comment into account, I'm starting to think that it's worth differentiating between normal graceful shutdowns and the immediate forced shutdown. That would clear up the code path confusion and give us higher quality information about what's happening with the process.

[ktff]

Going with the theme of past tense, the points of emissions have been moved to reflect this.

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Regarding the shutdown event, I've added a fix to the internal_metrics source because, as it is, we almost for certain won't capture it before we shutdown the source. So I've added a fix for it.

@lukesteensen this ties into immediate forced shutdown, it is highly unlikely that any event emitted on that path will be propagated further than internal_metrics source even with the fix. So I think it's better without as it would be unreliable.

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There are also errors in main which we could turn into events.