fix: use more intuitive envvars to manage OTLP and PostHog telemetry settings

[uinstinct]

Description

Re-opens #8328

resolves CON-4878

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• [] The relevant docs, if any, have been updated or created
• [] The relevant tests, if any, have been updated or created

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Summary by cubic

Switch to clearer env vars to control telemetry: CONTINUE_TELEMETRY_ENABLED for PostHog events and CONTINUE_METRICS_ENABLED for OTLP metrics, with smarter DNS checks and warnings when telemetry is blocked. Addresses Linear CON-4878.

• Migration

  • PostHog: use CONTINUE_TELEMETRY_ENABLED=1|0; falls back to CONTINUE_ALLOW_ANONYMOUS_TELEMETRY if unset.
  • OTLP metrics: use CONTINUE_METRICS_ENABLED=1|0; falls back to CONTINUE_CLI_ENABLE_TELEMETRY and still requires OTEL_* endpoints.

• Bug Fixes

  • Treat DNS resolutions to 0.0.0.0 and 127.* as blocked and skip telemetry.
  • Show a clear warning when telemetry is blocked by the network.
  • Add tests for blocked and valid DNS cases.

^{Written for commit 64dbb9a. Summary will update automatically on new commits.}

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Documentation Updates

Added documentation for the new telemetry environment variables:

Changes made:

• extensions/cli/spec/telemtry.md: Added CONTINUE_TELEMETRY_ENABLED as the primary variable for controlling PostHog telemetry, with CONTINUE_ALLOW_ANONYMOUS_TELEMETRY noted as legacy but still supported
• extensions/cli/spec/otlp-metrics.md: Added CONTINUE_METRICS_ENABLED as the primary variable for controlling OTLP metrics collection, with CONTINUE_CLI_ENABLE_TELEMETRY noted as legacy but still supported

Both new variables:

• Take precedence over the legacy variables
• Support explicit 1 (enable) and 0 (disable) values
• Provide more intuitive naming aligned with their purpose

[cubic-dev-ai]

No issues found across 3 files

[RomneyDa]

@sestinj could you do a final stamp on this one?

[RomneyDa]

@uinstinct could you update docs for this as well? I think docs have the old ones