Respect global config for OTel tracing and metrics enabled

[gkatz2]

Summary

• thv config otel set-tracing-enabled false was silently ignored by thv run because getTelemetryFromFlags had no fallback for TracingEnabled or MetricsEnabled, and buildRunConfig bypassed the fallback entirely for the proxy runner config
• Users could not globally disable telemetry without passing CLI flags on every invocation

Fixes #4323

Type of change

• Bug fix

Test plan

• Unit tests (task test)
• Linting (task lint-fix)
• Manual testing (describe below)

Deployed a local build and verified:

1. With global config tracing-enabled: false and metrics-enabled: false, thv run produces a runconfig with null telemetry (no telemetry initialized)
2. With explicit --otel-tracing-enabled=true, CLI flag correctly overrides global config
3. config.yaml now shows tracing-enabled: false instead of silently dropping the field

Changes

File	Change
pkg/config/config.go	Change TracingEnabled/MetricsEnabled from bool with omitempty to *bool without omitempty, matching the existing UseLegacyAttributes pattern
cmd/thv/app/run_flags.go	Add config fallback for both fields in getTelemetryFromFlags; use resolved values in buildRunConfig; return nil from createTelemetryConfig when both signals are disabled
cmd/thv/app/otel.go	Update setters/getters/unsetters for *bool
cmd/thv/app/run_flags_test.go	Update existing tests and add 4 new cases for tracing/metrics fallback

Does this introduce a user-facing change?

thv config otel set-tracing-enabled and set-metrics-enabled now take effect when starting servers with thv run. Previously these settings were silently ignored.

Special notes for reviewers

The *bool pattern (nil = never set, false = explicitly disabled) is necessary because the CLI defaults for these flags are true, unlike Insecure and EnablePrometheusMetricsPath which default to false. A struct-level comment on OpenTelemetryConfig explains this distinction.

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