envoyMetricsService Istio Histograms are Missing +Inf Upper Bound

**Title**: envoyMetricsService Istio Histograms are Missing +Inf Upper Bound

**Description**:
This is a problem with istio/proxyv2

With a normal sidecar/Kiali/Prometheus setup the Istio histogram metrics (ex. `istio_request_duration_milliseconds_bucket`) include the `+Inf` bucket.

With the metrics received by the gRPC envoyMetricsService the +Inf bucket is missing. This is a major issue for the [histogram_quantile](https://prometheus.io/docs/prometheus/latest/querying/functions/#histogram_quantile) Prometheus function.
>The highest bucket must have an upper bound of +Inf. (Otherwise, NaN is returned.)

For now I'll just convert the highest upper bound to +Inf in my metrics service.

[optional *Relevant Links*:]
https://istio.io/latest/docs/reference/config/istio.mesh.v1alpha1/#RemoteService

**Expected behavior**

It should include the `+Inf` bucket. Here's what I receive right now:

>[histogram: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: bucket: > timestamp_ms:1595617404646 ]

**Steps to reproduce the bug**

Run a gRPC envoy metrics service with an Istio sidecar and print the metrics received.

**Version (include the output of `istioctl version --remote` and `kubectl version` and `helm version` if you used Helm)**
```
client version: 1.6.4
control plane version: 1.6.4
data plane version: 1.6.4 (12 proxies)

Client Version: version.Info{Major:"1", Minor:"17", GitVersion:"v1.17.5", GitCommit:"e0fccafd69541e3750d460ba0f9743b90336f24f", GitTreeState:"clean", BuildDate:"2020-04-16T11:44:03Z", GoVersion:"go1.13.9", Compiler:"gc", Platform:"darwin/amd64"}
Server Version: version.Info{Major:"1", Minor:"17", GitVersion:"v1.17.9+IKS", GitCommit:"dbe4e8a5c169e0a6a8a9b7b183561fa61e161985", GitTreeState:"clean", BuildDate:"2020-07-16T01:30:57Z", GoVersion:"go1.13.9", Compiler:"gc", Platform:"linux/amd64"}
```