RPC: mention metrics on system-specific conventions

[lmolkova]

We include metrics section in DB-specific conventions, e.g. https://github.com/open-telemetry/semantic-conventions/blob/main/docs/db/mysql.md#metrics

This allows us to specify system name on metrics and also document if there are any details when implementing metrics for this system. E.g. JSON-RPC metrics might include jsonrpc.protocol.version attribute and after #3317 we'd probably add url.scheme for grpc.

So we should

• add metrics section to JSON-RPC, gRPC and Connect-RPC docs
• it should be similar to "{Protocol} instrumentations SHOULD collect metrics according to the general Semantic Conventions for RPC Metrics.\n rpc.system.name MUST be set to {"protocol"}.
• update table of content in each doc