kms: add Prometheus metrics endpoint

[kvinwang]

Summary

• add GET /metrics to dstack-kms in Prometheus text format
• expose dstack_kms_attestation_requests_total and dstack_kms_attestation_failures_total counters
• count attestation RPCs through a Rocket response fairing so pre-handler failures are included

Test

• rustfmt --edition 2021 --check kms/src/main.rs kms/src/main_service.rs
• cargo check -p dstack-kms

[Copilot]

Pull request overview

Adds a Prometheus-compatible metrics endpoint to the dstack-kms Rocket service and instruments attestation-related pRPC calls with in-process counters.

Changes:

• Add KmsMetrics counters to KmsState and a Prometheus text renderer.
• Expose GET /metrics returning Prometheus text format.
• Record attestation request/failure counts via a Rocket on_response fairing to include pre-handler failures.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
kms/src/main_service.rs	Introduces KmsMetrics in shared state and renders Prometheus exposition text.
kms/src/main.rs	Adds /metrics route and a response fairing to record attestation metrics for selected pRPC methods.

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