WIP - fix(stats): Skip group calculations if we don't have instances

[xiu]

When playing around with #627, I noticed a panic when running stats against the custom registry:

✔ `acme` semconv registry `tests/custom_registry` loaded (177 files)
Semantic Convention Registry Stats:
  - Total number of files: 177
✔ `acme` semconv registry resolved
Resolved Telemetry Schema Stats:
Registry
  - 2 groups
    - 0 Metrics
      - Total number of attributes: 0
        - [(attributes card: frequency), ...]: []

thread 'main' panicked at src/registry/stats.rs:198:13:
attempt to divide by zero
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Test

cargo run registry stats -r tests/custom_registry.

Expected behavior

No panic

[xiu]

Following https://github.com/open-telemetry/weaver/blob/17d6609d44ffac28c290380ecbca276928d8657e/CONTRIBUTING.md, I'm having an issue running just:

cargo machete
Analyzing dependencies of crates in this directory...
cargo-machete found the following unused dependencies in this directory:
weaver_resolved_schema -- ./crates/weaver_resolved_schema/Cargo.toml:
        serde_yaml

If you believe cargo-machete has detected an unused dependency incorrectly,
you can add the dependency to the list of dependencies to ignore in the
`[package.metadata.cargo-machete]` section of the appropriate Cargo.toml.
For example:

[package.metadata.cargo-machete]
ignored = ["prost"]

You can also try running it with the `--with-metadata` flag for better accuracy,
though this may modify your Cargo.lock files.

Done!
error: Recipe `pre-push-check` failed on line 17 with exit code 1

Not sure whether that is intended.

[lquerel]

Fixed in #697

[xiu]

Thanks!