[vnet] feat: SSH usage reporting

[nklaassen]

This PR adds usage reporting for VNet SSH sessions used via Connect.

[ravicious]

Now that I think about it, OnNewConnection ceased to be a good name and the comment makes it sound like it's being called for each new connection.

This name is used in many places so renaming it might be a bit of a pain, if you don't feel like taking care of this right now, then could you update the comments at least?

$ rg -B 2 "func.*OnNewConnection" -g 'lib/vnet/**' -g 'lib/teleterm/vnet/**' -g 'tool/tsh/**'
tool/tsh/common/vnet_client_application.go
142-// OnNewConnection gets called before each VNet connection. It's a noop as tsh doesn't need to do
143-// anything extra here.
144:func (p *vnetClientApplication) OnNewConnection(_ context.Context, _ *vnetv1.AppKey) error {

lib/vnet/client_application_service_client.go
136-
137-// OnNewConnection reports a new TCP connection to the target app.
138:func (c *clientApplicationServiceClient) OnNewConnection(ctx context.Context, appKey *vnetv1.AppKey) error {

lib/vnet/client_application_service.go
220-// OnNewConnection gets called whenever a new connection is about to be
221-// established through VNet for observability.
222:func (s *clientApplicationService) OnNewConnection(ctx context.Context, req *vnetv1.OnNewConnectionRequest) (*vnetv1.OnNewConnectionResponse, error) {

lib/vnet/app_provider.go
76-
77-// OnNewConnection reports a new TCP connection to the target app.
78:func (p *appProvider) OnNewConnection(ctx context.Context, appKey *vnetv1.AppKey) error {

lib/vnet/vnet_test.go
571-}
572-
573:func (p *fakeClientApp) OnNewConnection(_ context.Context, _ *vnetv1.AppKey) error {
--
1045-// TestOnNewConnection tests that the client applications OnNewConnection method
1046-// is called when a user connects to a valid TCP app.
1047:func TestOnNewConnection(t *testing.T) {

lib/vnet/app_handler.go
173-}
174-
175:func (m *localProxyMiddleware) OnNewConnection(ctx context.Context, lp *alpnproxy.LocalProxy) error {

lib/teleterm/vnet/service.go
549-// event. This is to mimic how Connect submits events for its app gateways. This lets us compare
550-// popularity of VNet and app gateways.
551:func (p *clientApplication) OnNewConnection(ctx context.Context, appKey *vnetv1.AppKey) error {

teleport/proto/teleport/lib/vnet/v1/client_application_service.proto

Lines 47 to 49 in 63be404

	// OnNewConnection gets called whenever a new connection is about to be
	// established through VNet for observability.
	rpc OnNewConnection(OnNewConnectionRequest) returns (OnNewConnectionResponse);

[nklaassen]

i did the rename

[nklaassen]

friendly ping @capnspacehook or @probakowski

[rosstimothy]

Not sure that it matters much since the context only appears to be used for logging in the ReportSSHSession implementation, but you could use context.WithoutCancel to ensure any contextual information is forwarded without risking the request context interrupting the usage reporting.