tests: TestExportMetrics_conversionFromViewData doesn't work with proxies

[jamie-digital]

Please answer these questions before submitting a bug report.

What version of the Exporter are you using?

v0.6.0

What version of OpenCensus are you using?

N/A

What version of Go are you using?

go version go1.13 darwin/amd64

What did you do?

If possible, provide a recipe for reproducing the error.

$ HTTP_PROXY=http://127.0.0.1:1234/ go test
--- FAIL: TestExportMetrics_conversionFromViewData (0.16s)
    viewdata_to_metrics_test.go:146: Got:
        null
        Want:
        [
          {
            "node": {
              "identifier": {
                "pid": 74790,
                "start_timestamp": {
                  "seconds": 1567689400,
                  "nanos": 12477000
                }
              },
              "library_info": {
                "language": 4,
                "exporter_version": "0.0.1",
                "core_library_version": "0.22.0"
              },
              "service_info": {}
            },
            "resource": {}
          },
          {
            "metrics": [
              {
                "metric_descriptor": {
                  "name": "ocagent.io/latency",
                  "description": "The latency of the various methods",
                  "unit": "ms",
                  "type": 4
                },
                "timeseries": [
                  {
                    "start_timestamp": {
                      "seconds": 1543160298,
                      "nanos": 997
                    },
                    "points": [
                      {
                        "timestamp": {
                          "seconds": 1543160298,
                          "nanos": 100000997
                        },
                        "Value": {
                          "Int64Value": 4
                        }
                      }
                    ]
                  }
                ]
              }
            ],
            "resource": {}
          }
        ]
FAIL
exit status 1
FAIL	contrib.go.opencensus.io/exporter/ocagent	3.289s

What did you expect to see?

Tests pass.

What did you see instead?

TestExportMetrics_conversionFromViewData fails, as the exporter attempts but fails to use an HTTP proxy for the host "".

Additional context

The test supplies an agent address with a port but no host:

	ocexp, err := NewExporter(
		WithInsecure(),
		WithAddress(":"+agentPortStr),      // <- no host here
		WithReconnectionPeriod(reconnectionPeriod),
	)

If HTTP_PROXY is set in the env, grpc (I assume) attempts to use an HTTP proxy. As the HTTP proxy code (https://github.com/golang/net/blob/master/http/httpproxy/proxy.go#L256) treats an empty host as an error, it isn't possible to set NO_PROXY to ignore the proxy for requests with no host. As a result, the exporter is unable to export metrics, causing TestExportMetrics_conversionFromViewData to fail.

The simplest fix would be to specify localhost or similar as the host.