No indication of cargo metadata being run

[lnicola]

When we open a project, we run cargo metadata, then do something with its output, but there's no progress being shown before the "workspace loaded" notification. Loading the project is usually fast (a couple of seconds or less), but cargo metadata will sometimes want to update the registry index and download the dependencies.

Updating the registry can be especially slow, so we should put up a progress indicator just in case. As for knowing the current status, it prints text, not JSON:

warning: please specify `--format-version` flag explicitly to avoid compatibility problems
    Updating crates.io index
  Downloaded fuchsia-zircon v0.3.3
  Downloaded digest v0.8.1
  Downloaded time-macros-impl v0.1.0
[snip]
  Downloaded proc-macro-error v0.4.9
  Downloaded winapi v0.3.8
  Downloaded 168 crates (16.0 MB) in 1.67s (largest was `winapi-x86_64-pc-windows-gnu` at 2.9 MB)
{"packages":[{"name":"thread_local","version":"1.0.1","id":"thread_local 1.0.1 (registry+https://github.com/rust-lang/crates.io-index)","license":"Apache-2.0/MIT","license_file":null,"description":"Per-object thread-local storage","source":"registry+https://github.com/rust-lang/crates.io-index","

Related to #3283.

[lnicola]

Triage: this still happens. Nuke ~/.cargo/registry and load a project to reproduce.