fix: metrics.json could use more data

[Mr-Neutr0n]

Fixes #685

Added a small, documented needs_overview (total/external/local/by_type) to the generic metrics.json, updated its JSON schema, added tests, and documented it — the issue's guidance (no arbitrary data, hard to remove fields) was treated as instruction, not followed literally into an arbitrary dump.

Could not run the suite locally: /config/init.py:2234. Fork CI needs a maintainer approval to run, so this branch has no test signal yet.

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[github-actions]

License Check Results

🚀 The license check job ran with the Bazel command:

bazel run --lockfile_mode=error //src:license-check

Status: ⚠️ Needs Review

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[License Check Output]
Extracting Bazel installation...
Starting local Bazel server (8.6.0) and connecting to it...
INFO: Invocation ID: bdeaaf86-66b3-42c5-875c-26191f443d73
Computing main repo mapping: 
Loading: 
Loading: 0 packages loaded
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Loading: 0 packages loaded
    currently loading: src
WARNING: Target pattern parsing failed.
ERROR: Skipping '//src:license-check': no such target '//src:license-check': target 'license-check' not declared in package 'src' defined by /home/runner/work/docs-as-code/docs-as-code/src/BUILD
ERROR: no such target '//src:license-check': target 'license-check' not declared in package 'src' defined by /home/runner/work/docs-as-code/docs-as-code/src/BUILD
INFO: Elapsed time: 6.929s
INFO: 0 processes.
ERROR: Build did NOT complete successfully
ERROR: Build failed. Not running target

[AlexanderLanin]

Hi @Mr-Neutr0n thanks for the contribution! I didn't look deeply into it yet, but doesn't it overlap with metrics_by_type?