docs(ai): add cost calculation docs

[shellmayr]

• Add a new page in the agent monitoring documentation that outlines where we get the cost data from and how the cost is calculated
• Add a new include that is used in manual instrumentation docs for Python, Node and Ruby that points out some gotchas with cost and token accounting
• Point out which costs we don't account for, especially non-token-based costs

Closes TET-1868

[linear]

TET-1868 Document cost calculation stuff that we're missing - Pricing for server side tools

[vercel]

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Project	Deployment	Actions	Updated (UTC)
develop-docs	Ready	Preview, Comment	Feb 6, 2026 0:27am
sentry-docs	Ready	Preview, Comment	Feb 6, 2026 0:27am

[sentry]

Bug: A link in the AI agent cost documentation points to a non-existent page for JavaScript/Node.js, resulting in a 404 error for users.
_{Severity: MEDIUM}

Suggested Fix

Update the markdown link in docs/product/insights/ai/agents/costs.mdx and docs/product/insights/ai/agents/getting-started.mdx from [JavaScript](/platforms/javascript/guides/node/ai-agent-monitoring/#token-usage-and-cost-gotchas) to [JavaScript](/platforms/javascript/common/ai-agent-monitoring/#token-usage-and-cost-gotchas).

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent.
Verify if this is a real issue. If it is, propose a fix; if not, explain why it's not
valid.

Location: docs/product/insights/ai/agents/costs.mdx#L69

Potential issue: The cost calculation documentation at
`docs/product/insights/ai/agents/costs.mdx` and the getting-started guide contain a
broken link to the JavaScript/Node.js AI agent monitoring documentation. The link
incorrectly points to `/platforms/javascript/guides/node/ai-agent-monitoring/`, which
does not exist. The correct location for this documentation is
`/platforms/javascript/common/ai-agent-monitoring/`. As a result, users trying to access
the token cost documentation for JavaScript/Node.js will encounter a 404 error,
preventing them from finding important information about token accounting.

_{Did we get this right? 👍 / 👎 to inform future reviews.}