Tool costs are recorded with no currency — attach a unit to gen_ai.usage.total_cost #1462

The per-trace cost has no unit of account

AgentOps records a monetary cost for LLM/tool operations on spans, e.g. when a
decorated tool sets a cost:

# agentops/sdk/decorators/factory.py:392 (and 411, 432, 464)
span.set_attribute(SpanAttributes.LLM_USAGE_TOOL_COST, cost)

with the attribute key:

# agentops/semconv/span_attributes.py:68
LLM_USAGE_TOOL_COST = "gen_ai.usage.total_cost"

The value is a float produced from the tokencost price tables
(app/opentelemetry-collector/builder/costs/__init__.py models cost as a Decimal
"for precise currency calculations", loading per-token USD prices from
model_prices.json). So today the number is always implied USD — but the span
carries no unit of account. Anyone who self-hosts AgentOps, ingest traces billed
in a non-USD currency, or sinks spans into a downstream cost dashboard or billing
reconciliation step cannot tell what currency the number is in, and cross-account
or cross-deployment cost comparison is impossible. It is observability data with
no currency attached to it.

The standard direction

The OpenTelemetry GenAI semantic-conventions working group is standardizing
exactly this gap (there is no OTel convention for GenAI cost today; each vendor
rolls its own shape). The open proposal
add per-operation cost conventions (gen_ai.usage.cost.*) #443
defines:

• gen_ai.usage.cost.amount (double) — the monetary cost at recording time
• gen_ai.usage.cost.currency (string) — the ISO 4217 code, conditionally required when amount is set
• gen_ai.usage.cost.source (enum provider | pricing_table | estimate) — provenance

So the attribute a span should carry beside its cost amount is a currency
companion, and the natural place to add it is the semantic-conventions store where
LLM_USAGE_TOOL_COST already lives.

Proposed change (small, additive)

1. In agentops/semconv/span_attributes.py next to LLM_USAGE_TOOL_COST (line 68):

   # currency companion, aligned with OTel GenAI semconv PR #443
   LLM_USAGE_TOOL_COST_CURRENCY = "gen_ai.usage.cost.currency"

2. In agentops/sdk/decorators/factory.py, on the line after each of the four
   LLM_USAGE_TOOL_COST set sites (392, 411, 432, 464), set the companion where a
   cost is set:

   span.set_attribute(SpanAttributes.LLM_USAGE_TOOL_COST_CURRENCY, "USD")

   (USD because the tokencost price tables price in USD today.)
   Keep the cost value itself and the charged amount untouched.
3. A focused test asserting that a tool span with a cost also carries the
   currency attribute equal to "USD"; the existing test suite stays green.

Why it helps

• The cost number becomes interpretable: it names its unit of account.
• Aligns the vendor extension with the emerging OTel GenAI standard, where
  currency is required whenever an amount is set.
• Lets a self-hosted operator attribute cost in any ISO 4217 currency (USD today,
  others later) without changing the amount logic — a property that matters to any
  per-trace cost-accounting or billing workflow that consumes AgentOps spans.

Contained to the decorator tool path, additive, one attribute key, no change to
the cost value or existing behavior. Happy to open a PR with the test if this
scope looks right.