internal: add json `tracing` Layer for profiling startup
On `buck2/integrations/rust-project`, this results in the following being printed:
```json
{"name":"discover_command","elapsed_ms":18703}
{"name":"parallel_prime_caches","elapsed_ms":0}
{"name":"vfs_load","elapsed_ms":5895}
{"name":"vfs_load","elapsed_ms":547}
{"name":"parallel_prime_caches","elapsed_ms":23}
{"name":"parallel_prime_caches","elapsed_ms":84}
{"name":"parallel_prime_caches","elapsed_ms":5819}
```
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