Add test for Prometheus Receiver to verify timeseries with empty label values are supported

[mustafain117]

Is your feature request related to a problem? Please describe.
We want to ensure that the Prometheus receiver supports timeseries with empty label values. According to Prometheus documentation (https://prometheus.io/docs/concepts/data_model/), a label with an empty label value is considered equivalent to a label that does not exist.
Describe the solution you'd like
We propose the following solution:

Add a test that verifies timeseries with empty label values are supported.

The test will create a timeseries with empty label value.

Note that a label with an empty value is considered equivalent to a label that does not exist, i.e. Prometheus receiver should drop the label.

Case 1: Metric data with empty label value and no other label:

# HELP test_gauge0 This is my gauge
# TYPE test_gauge0 gauge
test_gauge0{label1=""} 10

Expected Result: MetricDescriptor for test_gauge0 should not have “label1” as a LabelKey.

Case 2: Metric with two time-series, one has a value for the label "foo" whereas the other doesn’t:

# HELP gauge_test some test gauges.
# TYPE gauge_test gauge
gauge_test{id="1",foo="bar"} 1.0
gauge_test{id="2",foo=""}    2.0

Expected Result:

• MetricDescriptor for gauge_test should have “foo” as a LabelKey
• Timeseries for gauge_test{id=“2”, foo=“”} should have
  metricspb.LabelValue{ Value: “”, HasValue: false}

The corresponding OTLP format of the above case should be:

metrics := []*metricspb.Metric{
  {
    MetricDescriptor: &metricspb.MetricDescriptor{
      Name:      "gauge_test",
      Type:      metricspb.MetricDescriptor_GAUGE_DOUBLE,
      LabelKeys: []*metricspb.LabelKey{{Key: "id"}, {Key: "foo"}}},
    Timeseries: []*metricspb.TimeSeries{
      {
        StartTimestamp: nil,
        LabelValues:    []*metricspb.LabelValue{{Value: "1", HasValue: true}, {Value: "bar", HasValue: true}},
        Points: []*metricspb.Point{
          {Timestamp: currentTimestamp, Value: &metricspb.Point_DoubleValue{DoubleValue: 1.0}},
        },
      },
      {
        StartTimestamp: nil,
        LabelValues:    []*metricspb.LabelValue{{Value: "2", HasValue: true}, {Value: "", HasValue: false}},
        Points: []*metricspb.Point{
          {Timestamp: currentTimestamp, Value: &metricspb.Point_DoubleValue{DoubleValue: 2.0}},
        },
      },
    },
  },
}

Additional context
Related to open-telemetry/wg-prometheus#57

CC: @alolita @Aneurysm9

[mustafain117]

@alolita please assign this issue to me.