Add token usage metrics to InstrumentedModel

[alexmojaki]

Implements https://opentelemetry.io/docs/specs/semconv/gen-ai/gen-ai-metrics/#metric-gen_aiclienttokenusage

Example query in logfire:

select sum(histogram_sum)                 as "Total tokens",
       attributes ->> 'gen_ai.token.type' as "Token type"
from metrics
where metric_name = 'gen_ai.client.token.usage'
group by "Token type"

Not in a hurry to add docs since this doesn't feel user friendly, would rather point to a managed dashboard template or metrics being aggregated in spans.

[github-actions]

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