make handler proces more robust

There was a timing window when the handler grpc client was available, but before it was ready to send back metrics and if metrics were gathered there, the whole HTTP metrics response would be something like this
```
An error has occurred while serving metrics:

[from Gatherer #2] rpc error: code = Unavailable desc = connection error: desc = "transport: Error while dialing: dial unix /tmp/EGH_ME3wf4gMCrac/service_rpc.sock: connect: no such file or directory"
```

That came from prometheus.registry.go:761, which sends back an error from the `Gatherers.Gather()` invocation which the Prometheus HTTP handler then decides to return.   What I've done is to quietly swallow errors from the handler (after logging) and simply return empty metrics objects and no error.

pkg/service/process.go

@@ -201,18 +201,18 @@ func getSocketAddress(handlerTmpDir string) string {
 // Gather implements the prometheus.Gatherer interface on server-side to allow aggregation of handler metrics
 func (p *Process) Gather() ([]*dto.MetricFamily, error) {
 	// Get the metrics from the handler via IPC
-	logger.Debugw("gathering metrics from handler process", "handlerID", p.handlerID)
+	logger.Debugw("gathering metrics from handler process", "egress_id", p.req.EgressId)
 	metricsResponse, err := p.grpcClient.GetMetrics(context.Background(), &ipc.MetricsRequest{})
 	if err != nil {
-		logger.Errorw("Error obtaining metrics from handler", err)
-		return make([]*dto.MetricFamily, 0), err
+		logger.Warnw("Error obtaining metrics from handler, skipping", err, "egress_id", p.req.EgressId)
+		return make([]*dto.MetricFamily, 0), nil // don't return an error, just skip this handler
 	}
 	// Parse the result to match the Gatherer interface
 	parser := &expfmt.TextParser{}
 	families, err := parser.TextToMetricFamilies(strings.NewReader(metricsResponse.Metrics))
 	if err != nil {
-		logger.Errorw("Error parsing metrics from handler", err)
-		return make([]*dto.MetricFamily, 0), err
+		logger.Warnw("Error parsing metrics from handler, skipping", err, "egress_id", p.req.EgressId)
+		return make([]*dto.MetricFamily, 0), nil // don't return an error, just skip this handler
 	}
 
 	// Add an egress_id label to every metric all the families, if it doesn't already have one