Prevent from attempting to create metrics with unspecified MetricKind

[ygonzal6]

Attempting to create metrics with unspecified metricKind floods GCP logs with the following error: "Request was missing field metricDescriptor.metricKind: The descriptor does not have the metric kind set." This will happen when metric type is of TypeSummary.

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[dashpole]

/gcbrun please add unit tests

[dashpole]

/gcbrun

[dashpole]

=== RUN   TestMetricTypeToMonitoringMetricKind
--- FAIL: TestMetricTypeToMonitoringMetricKind (0.00s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
    panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xe5e139]

goroutine 2541 [running]:
testing.tRunner.func1.2({0xf40920, 0x218d8e0})
    /usr/local/go/src/testing/testing.go:1209 +0x36c
testing.tRunner.func1()
    /usr/local/go/src/testing/testing.go:1212 +0x3b6
panic({0xf40920, 0x218d8e0})
    /usr/local/go/src/runtime/panic.go:1047 +0x266
contrib.go.opencensus.io/exporter/stackdriver.TestMetricTypeToMonitoringMetricKind(0xc000225a00)
    /workspace/metrics_test.go:433 +0x459
testing.tRunner(0xc000225a00, 0x106a290)
    /usr/local/go/src/testing/testing.go:1259 +0x230
created by testing.(*T).Run
    /usr/local/go/src/testing/testing.go:1306 +0x727
FAIL    contrib.go.opencensus.io/exporter/stackdriver   8.212s

[dashpole]

/gcbrun