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.

opamp-go/client/client.go

Lines 11 to 83 in 9efb265

	type OpAMPClient interface {
	// Start the client and begin attempts to connect to the Server. Once connection
	// is established the client will attempt to maintain it by reconnecting if
	// the connection is lost. All failed connection attempts will be reported via
	// OnConnectFailed callback.
	//
	// SetAgentDescription() MUST be called before Start().
	//
	// Start may immediately return an error if the settings are incorrect (e.g. the
	// serverURL is not a valid URL).
	//
	// Start does not wait until the connection to the Server is established and will
	// likely return before the connection attempts are even made.
	//
	// It is guaranteed that after the Start() call returns without error one of the
	// following callbacks will be called eventually (unless Stop() is called earlier):
	// - OnConnectFailed
	// - OnError
	// - OnRemoteConfig
	//
	// Start should be called only once. It should not be called concurrently with
	// any other OpAMPClient methods.
	Start(ctx context.Context, settings types.StartSettings) error
	// Stop the client. May be called only after Start() returns successfully.
	// May be called only once.
	// After this call returns successfully it is guaranteed that no
	// callbacks will be called. Stop() will cancel context of any in-fly
	// callbacks, but will wait until such in-fly callbacks are returned before
	// Stop returns, so make sure the callbacks don't block infinitely and react
	// promptly to context cancellations.
	// Once stopped OpAMPClient cannot be started again.
	Stop(ctx context.Context) error
	// SetAgentDescription sets attributes of the Agent. The attributes will be included
	// in the next status report sent to the Server. MUST be called before Start().
	// May be also called after Start(), in which case the attributes will be included
	// in the next outgoing status report. This is typically used by Agents which allow
	// their AgentDescription to change dynamically while the OpAMPClient is started.
	// May be also called from OnMessage handler.
	//
	// nil values are not allowed and will return an error.
	SetAgentDescription(descr *protobufs.AgentDescription) error
	// AgentDescription returns the last value successfully set by SetAgentDescription().
	AgentDescription() *protobufs.AgentDescription
	// SetHealth sets the health status of the Agent. The AgentHealth will be included
	// in the next status report sent to the Server. MAY be called before or after Start().
	// May be also called after Start().
	// May be also called from OnMessage handler.
	//
	// nil health parameter is not allowed and will return an error.
	SetHealth(health *protobufs.AgentHealth) error
	// UpdateEffectiveConfig fetches the current local effective config using
	// GetEffectiveConfig callback and sends it to the Server.
	// May be called anytime after Start(), including from OnMessage handler.
	UpdateEffectiveConfig(ctx context.Context) error
	// SetRemoteConfigStatus sets the current RemoteConfigStatus.
	// LastRemoteConfigHash field must be non-nil.
	// May be called anytime after Start(), including from OnMessage handler.
	// nil values are not allowed and will return an error.
	SetRemoteConfigStatus(status *protobufs.RemoteConfigStatus) error
	// SetPackageStatuses sets the current PackageStatuses.
	// ServerProvidedAllPackagesHash must be non-nil.
	// May be called anytime after Start(), including from OnMessage handler.
	// nil values are not allowed and will return an error.
	SetPackageStatuses(statuses *protobufs.PackageStatuses) error
	}

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.