feat(metrics): Add "unknown" for dest labels on local replies

[nojnhuh]

Description:
Because destination_* tags are generated at the request's destination,
requests that invoke a local response from Envoy have no destination_*
values. This change modifies Envoy to add the expected header keys with
the value "unknown" to stay consistent with the behavior for requests
that don't invoke a local reply.

---

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. (docs(metrics): Add docs for WASM stats #2503) docs(metrics): Add docs for WASM stats
4. (feat(metrics): Add WASM feature flag #2517) feat(metrics): Add WASM feature flag
5. (feat(metrics): Add stats.wasm to proxy config #2546) feat(metrics): Add stats.wasm to proxy config
6. (feat(injector): Add name, workload name, workload kind to PodMetadata #2554) feat(injector): Add name, workload name, workload kind to PodMetadata
7. (feat(metrics): Add source labels to WASM metrics #2571) feat(metrics): Add source labels to WASM metrics
8. (feat(metrics): Add dest labels to WASM metrics #2574) feat(metrics): Add dest labels to WASM metrics
9. (YOU ARE HERE) 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 remaining changes can be found here: main...nojnhuh:wasm-metrics

Affected area:

• New Functionality [ ]
• Documentation [ ]
• Install [ ]
• Control Plane [X]
• CLI Tool [ ]
• Certificate Management [ ]
• Networking [ ]
• Metrics [X]
• 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

[codecov-io]

Codecov Report

Merging #2587 (7ed9462) into main (1583cf1) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2587   +/-   ##
=======================================
  Coverage   63.20%   63.20%           
=======================================
  Files         154      154           
  Lines        7153     7167   +14     
=======================================
+ Hits         4521     4530    +9     
- Misses       2615     2620    +5     
  Partials       17       17           

Flag	Coverage Δ
unittests	63.20% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pkg/envoy/lds/connection_manager.go	92.68% <100.00%> (+1.50%)	⬆️
pkg/configurator/validating_webhook.go	76.55% <0.00%> (-3.45%)	⬇️

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