Rollup merge of #140815 - yaahc:rustdoc-metrics, r=GuillaumeGomez

also export metrics from librustdoc

Addresses the issue mentioned here: [#t-docs-rs > metrics intitiative @ 💬](https://rust-lang.zulipchat.com/#narrow/channel/356853-t-docs-rs/topic/metrics.20intitiative/near/515714331)

The previous implementation only emitted metrics from rustc, but it turns out running `cargo doc` only calls `rustc` for dependencies, and not for the root crate being documented. We are planning to gather a sample dataset from docs.rs ci via `cargo doc` so as things stood this would not emit any metrics for any of the crates themselves that were published.

This change adds the same logic from `rustc_driver_impl` to `librustdoc` to also dump metrics at the end of its execution if they are enabled.

Note: The hash's generated by librustdoc will likely be completely different from the ones generated by rustc. This is because rustc is actually doing the various passes needed to fully calculate the stable version hash. My understanding of how rustdoc works is that the hashes generated will be working with partial information due to it only doing the work required to generate docs. The hashes will still be unique per crate and will work for the purposes of the metrics proof of concept, it would not be possible to correlate metrics generated by rustdoc with those generated by rustc for the same crate. This is fine for the purposes of the PoC but a future full implementation of metrics may want to address this issue.

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