Capability to set RequestInstanceUid flag in AgentToServer msg as per the spec

[Sairam-Arpavur-Natarajan]

I am trying to implement our use case of the Agent requesting the Server for the identifier. The specification supports it, but its yet implemented in the opamp-go. https://github.com/open-telemetry/opamp-spec/blob/main/specification.md#agenttoserverinstance_uid. I can contribute to the go implementation. Any pointers for the changes will be helpful.

[srikanthccv]

We need to add SetFlags on the OpAMPClient interface. And ClientCommon implementation to schedule to send it in the next message. https://github.com/open-telemetry/opamp-go/blob/9efb265ce3818016ed36ecfa0fc3ce86c5c1cf96/client/client.go#L11-L83

Just to note, If your server implementation sends a non-nil AgentIdentification field, the new instance ID is always honoured and will be set for subsequent messages.

[Sairam-Arpavur-Natarajan]

I can verify that behavior. But would like to follow the protocol and have the capability in Agent to specifically request new instance ID from the Server.