docs(metrics): Add docs for WASM stats

[nojnhuh]

Description:
This change adds documentation regarding the WASM-based Envoy extension
used to implement SMI metrics. User-facing docs have been added with
what metrics are generated and known gaps. Developer documentation
includes how to build the extension and a high-level description of how
it works.

---

This PR is part of several which together extend Envoy to generate custom
metrics needed to implement the SMI metrics spec (#984).
Here is a map of the PRs as they're currently planned:

1. (ref(injector): pass pod object to getEnvoySidecarContainerSpec #2479) ref(injector): pass pod object to getEnvoySidecarContainerSpec
2. (feat(metrics): add WASM metrics source #2488) feat(metrics): add WASM metrics source
3. (YOU ARE HERE) docs(metrics): Add docs for WASM stats
4. feat(metrics): Add WASM feature flag
5. feat(metrics): Add stats.wasm to workload pods
6. feat(injector): Add name, workload name, workload kind to PodMetadata
7. feat(metrics): Add source labels to WASM metrics
8. feat(metrics): Add dest labels to WASM metrics
9. feat(metrics): Add "unknown" for dest labels on local replies
10. feat(metrics): clean up WASM metrics in Prometheus config
11. feat(metrics): add flag to enable WASM metrics
12. tests(e2e): add WASM metrics test

The full set of changes can be found here: main...nojnhuh:wasm-metrics

Affected area:

• New Functionality [ ]
• Documentation [X]
• Install [ ]
• Control Plane [ ]
• CLI Tool [ ]
• Certificate Management [ ]
• Networking [ ]
• Metrics [ ]
• SMI Policy [ ]
• Security [ ]
• Tests [ ]
• CI System [ ]
• Performance [ ]
• Other [ ]

Please answer the following questions with yes/no.

• Does this change contain code from or inspired by another project? If so, did you notify the maintainers and provide attribution?
  No

[snehachhabria]

We should support these metrics for tcp as well given that OSM now allows TCP traffic. It doesn't need to be solved in this PR but something to make note of

[nojnhuh]

I agree. For this initial implementation I'll plan to only include compatibility with HTTP. I've opened #2506 to track making it applicable to all TCP traffic.