Deprecate redundant custom duration metrics in favor of OTEL MCP standard metrics

Summary

ToolHive now emits OTEL MCP semantic convention duration metrics alongside the original custom metrics. Two custom histogram metrics are redundant and should be deprecated and eventually removed.

Redundant Metrics

Both pairs measure the same duration in seconds using the same histogram bucket boundaries ([0.01, 0.02, 0.05, 0.1, 0.2, 0.5, 1, 2, 5, 10, 30, 60, 120, 300]). The standard metrics use richer OTEL MCP semconv attributes (mcp.method.name, network.transport, error.type, gen_ai.tool.name, etc.).

Proposed Deprecation Plan

1. Current release: Both custom and standard metrics emitted (status quo)
2. Next minor release: Log a deprecation warning at startup when the custom metrics are being consumed (if detectable), and document the deprecation in release notes
3. Following minor release: Remove the custom metrics entirely

Non-Redundant Metrics (No Action Needed)

These custom metrics have no direct OTEL MCP standard equivalent and should be retained:

• toolhive_mcp_requests (Counter) — no standard counter exists
• toolhive_mcp_tool_calls (Counter) — scoped convenience counter for tools/call
• toolhive_mcp_active_connections (UpDownCounter) — no standard gauge equivalent
• toolhive_vmcp_backend_requests (Counter) — no standard counter exists
• toolhive_vmcp_backend_errors (Counter) — no standard error counter
• toolhive_vmcp_backends_discovered (Gauge) — ToolHive-specific operational metric
• toolhive_vmcp_workflow_* (3 metrics) — composite workflows are a ToolHive concept

Related

• Parent issue: #3399 (OTEL telemetry improvements)
• #3764 (implement mcp.server.session.duration)
• #3765 (JSON-RPC error code capture in error.type)

Start by reading pkg/telemetry/middleware.go and pkg/vmcp/server/telemetry.go to compare the two custom duration metrics with their OTEL MCP replacements and confirm their consumers. Check the release-note process and determine how deprecation warnings can be detected; done means the agreed staged deprecation is documented and implemented without changing the non-redundant metrics.