feat(observability): toolkit and DB pool metrics (Trino, DataHub, S3, OAuth, database/sql)

[cjimti]

Part of #459.

Problem

Only MCP middleware and the apigateway outbound transport are instrumented today.
No visibility into Trino query volume / latency, DataHub fetch performance, S3 ops,
OAuth token issuance and refresh outcomes, or database/sql connection pool
saturation. Diagnostics for the rest of the platform are still blind.

Acceptance criteria

New OTel instruments registered in pkg/observability/metrics.go:

• Trino
  • trino_queries_total{status, query_kind}
  • trino_query_duration_seconds{query_kind}
  • trino_bytes_scanned_total{query_kind}
• DataHub
  • datahub_requests_total{operation, status}
  • datahub_request_duration_seconds{operation}
• S3
  • s3_operations_total{operation, status}
  • s3_operation_duration_seconds{operation}
• OAuth
  • oauth_token_issuance_total{grant_type, status}
  • oauth_token_refresh_total{status}
  • oauth_token_refresh_duration_seconds
• DB pool (via OTel observable gauges that tap (*sql.DB).Stats())
  • db_pool_open_connections{pool}
  • db_pool_in_use{pool}
  • db_pool_idle{pool}
  • db_pool_wait_count_total{pool}
  • db_pool_wait_duration_seconds_total{pool}

Plus:

• Status labels bounded: ok, client_err, server_err, upstream_err.
• DB pool gauges register exactly once per platform start via meter.RegisterCallback.
• Each toolkit records observations without changing public interfaces.
• Unit tests per instrument; integration test issues a query through each toolkit
  and asserts the corresponding metric increments.

Implementation notes

• Trino: instrument the query execution provider in pkg/query/trino/adapter.go.
• DataHub: instrument the semantic provider in pkg/semantic/datahub/adapter.go.
• S3: instrument the S3 toolkit call sites.
• OAuth: instrument pkg/oauth/server.go token endpoints.
• DB pool: add RegisterDBPool(*sql.DB, name string) on *observability.Metrics, called once per managed DB handle from cmd/mcp-data-platform/main.go. The pool label distinguishes platform DB vs OAuth store vs audit store if separate.
• All label cardinality bounded by configured connections / operations.

Files

• pkg/observability/metrics.go
• pkg/query/trino/adapter.go
• pkg/semantic/datahub/adapter.go
• S3 toolkit adapters
• pkg/oauth/server.go
• cmd/mcp-data-platform/main.go