Update MetricDescriptor comment to reflect the label key-value structure properly

[nilebox]

The comment on MetricDescriptor currently states that label keys are part of the MetricDescripror, while label values are defined in each data point:

opentelemetry-proto/opentelemetry/proto/metrics/v1/metrics.proto

Lines 60 to 61 in c44400d

	// | | ||label|label|...|label| |
	// | data|--+ ||key1 |key2 | |keyN | |

This has been changed in #102 to each data point having key-value pairs for labels:

opentelemetry-proto/opentelemetry/proto/metrics/v1/metrics.proto

Lines 245 to 246 in c44400d

	// The set of labels that uniquely identify this timeseries.
	repeated opentelemetry.proto.common.v1.StringKeyValue labels = 1;

, so this comment needs to be updated as well.

/cc @bogdandrutu