Add endpoint load metrics

[efimki]

Implement for V0 engine only for chat/completion and /completion

FIX #10086

• reporting of engine metrics in response headers when enabled through request header "endpoint-load-metrics-format"
• supported formats "text" and "json"
• No added load on response for default case
• No new computation in engine

Forked from #14906

[github-actions]

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[efimki]

This PR is exact copy of #14906 to merge the work started by @coolkp