pricing: add currency label and change the metric name to llm_model_cost_total

[tao12345666333]

What type of PR is this?

o11y: change the metric name to llm_model_cost_total and add a currency label

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #75

Release Notes: Yes/No

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[github-actions]

👥 vLLM Semantic Team Notification

The following members have been identified for the changed files in this PR and have been automatically assigned:

📁 config

Owners: @rootfs
Files changed:

• config/config.yaml

📁 src

Owners: @rootfs, @Xunzhuo, @wangchen615
Files changed:

• src/semantic-router/pkg/config/config.go
• src/semantic-router/pkg/extproc/response_handler.go
• src/semantic-router/pkg/metrics/metrics.go

📁 website

Owners: @Xunzhuo
Files changed:

• website/docs/api/router.md
• website/docs/getting-started/configuration.md

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This comment was automatically generated based on the OWNER files in the repository.