Add dual-semconv support to RPC metrics

[zeitlinger]

Part of #15932 — split into smaller reviewable PRs. This is PR 4 of 5 (depends on #16130).

• Add stable metric rpc.client.call.duration (seconds) alongside old rpc.client.duration (milliseconds)
• Add stable metric rpc.server.call.duration (seconds) alongside old rpc.server.duration (milliseconds)
• Update RpcMetricsAdvice with separate old/stable attribute key lists (rpc.system.name, rpc.response.status_code for stable)
• Add OLD_RPC_METHOD_CONTEXT_KEY infrastructure for correct rpc.method handling in dup mode
• Size metrics (rpc.*.request.size, rpc.*.response.size) remain old-only
• Tests validate all three modes: old-only, stable-only, and dup

#15871