Support SDK key on HTTP endpoints and RPCs / OTEL

[z4kn4fein]

Describe the purpose of your pull request

This PR contains the following:

• Ability to use SDK keys in addition to SDK IDs on all HTTP endpoints and RPCs for identifying a certain SDK within the Proxy.
• The current Prometheus-only observability solution was replaced with Open Telemetry. Some metric keys have been changed as official packages replaced most custom meters:
  • configcat_http_request_duration_seconds -> configcat_http_server_request_duration_seconds
  • configcat_grpc_rpc_duration_seconds -> configcat_rpc_server_duration_milliseconds
  • configcat_sdk_http_request_duration_seconds -> configcat_http_client_request_duration_seconds
• Ability to export metrics/traces to OTLP. (metrics remain available in Prometheus format)
• OFREP endpoints are enabled by default.

Security (only if applicable)

• n/a

Requirement checklist (only if applicable)

• I have covered the applied changes with automated tests.
• I have executed the full automated test set against my changes.
• I have validated my changes against all supported platform versions.
• I have read and accepted the contribution agreement.

[sonarqubecloud]

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