No metric distinguishes GraphQL from anything else #374

Background

src/metrics.ts has no GraphQL metrics, and src/middleware/httpMetrics.ts
sees GraphQL as a single route: POST /graphql. So every GraphQL request looks
identical on the dashboard whatever it asked for, and getPrice traffic is
invisible — the resolver increments price_requests_total, which does not say
it came from GraphQL.

Once L188 prices GraphQL, "how much GraphQL traffic is there and what is it
asking for" becomes a revenue question, not just an operational one.

What to build

• A counter of GraphQL operations by operation name and type (query,
  mutation, subscription).
• A histogram of resolver duration by field, for the expensive fields at least.
• A counter of GraphQL errors by category.
• Keep label cardinality bounded. Operation names come from the client and
  are therefore unbounded — fold unknown names into one label value rather than
  creating a series per name.

Acceptance criteria

• Operations are counted by name and type
• Resolver duration is recorded for the database-backed fields
• Errors are counted with a bounded set of categories
• No client-supplied string becomes an unbounded label
• Metrics appear on the existing /metrics endpoint

Drips Wave · Complexity: Easy · 100 points

Repo: Divinity-Labs/Lens. Node + TypeScript, Fastify, mercurius for
GraphQL at POST /graphql, Prisma on Postgres, Redis, Prometheus.

Run the tests: npm test. The tests are vitest, not jest. Running jest
here reports "66 suites, 0 tests" and means nothing. A single file is
npx vitest run src/__tests__/graphqlSubscription.test.ts.

Load test (prices) is already failing on main and has been for some
time. It is not your PR. Everything else should be green.

Read src/__tests__/graphqlSubscription.test.ts before you start. It is the
house pattern for testing this surface and it is good — it drives a real
subscription through a real server and asserts on what a client receives.

This service prices real assets on Stellar mainnet. A wrong number is worse
than a missing one, because the caller cannot tell.

Never elide an address, a contract id or a key in code, comments or test
fixtures. A previous batch shortened a contract id in an issue and five
contributors invented plausible-looking ids to fill the gap. Write it in full or
not at all.

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