Optimize LabelValues in metric proto

LabelValues is now a separate message.

Benchmarking results are below (Baseline is current `master`, Proposed is after this commit).
This change reduces CPU usage by about 18% for one-data-point timeseries encoding and reduces
memory consumption by about 7%.

```
===== Encoded sizes
Encoding                       Uncompressed  Improved        Compressed  Improved
Baseline/MetricOne              19150 bytes  [1.000], gziped 1600 bytes  [1.000]
Proposed/MetricOne              18550 bytes  [1.032], gziped 1601 bytes  [0.999]

Encoding                       Uncompressed  Improved        Compressed  Improved
Baseline/MetricSeries           43115 bytes  [1.000], gziped 6316 bytes  [1.000]
Proposed/MetricSeries           42515 bytes  [1.014], gziped 6285 bytes  [1.005]

goos: darwin
goarch: amd64
pkg: github.com/tigrannajaryan/exp-otelproto/encodings
BenchmarkEncode/Baseline/MetricOne-8           	      42	 125724579 ns/op
BenchmarkEncode/Proposed/MetricOne-8           	      62	 102677556 ns/op

BenchmarkEncode/Baseline/MetricSeries-8        	      16	 323098665 ns/op
BenchmarkEncode/Proposed/MetricSeries-8        	      19	 311684082 ns/op

BenchmarkDecode/Baseline/MetricOne-8           	      21	 274958910 ns/op	156696037 B/op	 4274000 allocs/op
BenchmarkDecode/Proposed/MetricOne-8           	      24	 268134546 ns/op	144696034 B/op	 3974000 allocs/op

BenchmarkDecode/Baseline/MetricSeries-8        	       8	 648421823 ns/op	339096032 B/op	 8374000 allocs/op
BenchmarkDecode/Proposed/MetricSeries-8        	       9	 637746497 ns/op	327096035 B/op	 8074000 allocs/op
```

opentelemetry/proto/metrics/v1/metrics.proto

@@ -104,13 +104,43 @@ message MetricDescriptor {
   repeated string label_keys = 5;
 }
 
+// LabelValues is a list of label string values.
+message LabelValues {
+  // Values of labels. Each element in this array is the value of the key defined at the
+  // corresponding index of MetricDescriptor.label_keys array. The number of elements
+  // in these 2 arrays must be the same. Note that values array must include empty
+  // strings and must also contain an empty string value for labels for which the
+  // value was originally unspecified.
+  repeated string values = 1;
+
+  // Some metric sources allow setting the value of a label to be equal not just to
+  // empty string but to an unspecified value, or in other words the value of a particular
+  // label defined in label_keys array may be missing in a particular timeseries.
+  // In order to support this concept and because ProtoBuf cannot distinguish empty string
+  // from unspecified string we store indexes of unspecified labels in "values" array in
+  // this field. This field contains only indexes of unspecified labels.
+  // This field should not contain duplicate numbers, numbers less than zero or
+  // numbers greater than or equal to the length of values array.
+  repeated int32 unspecified_value_indexes = 2;
+
+  // For example:
+  //   label_keys = {"a",      "b",             "c", "d", "e"}
+  //   values =     {"value1", "another value", "",  "",  "yet another string"}
+  //   unspecified_value_indexes = {2}
+  // Because unspecified_value_indexes field contains value 2 it means that the empty
+  // string at index 3 of values array must be interpreted as unspecified value and
+  // not as an empty string, which means label "c" has no value. As opposed to that
+  // empty value at index 3 is treated as normal empty string so label "d" has
+  // value of empty string.
+}
+
 // Int64TimeSeries is a list of data points that describes the time-varying values
 // of a int64 metric.
 message Int64TimeSeries {
   // The set of label values that uniquely identify this timeseries. Applies to
   // all points. The order of label values must match that of label keys in the
   // metric descriptor.
-  repeated LabelValue label_values = 1;
+  LabelValues label_values = 1;
 
   // The data points of this timeseries.
   repeated Int64Value points = 2;
@@ -122,7 +152,7 @@ message DoubleTimeSeries {
   // The set of label values that uniquely identify this timeseries. Applies to
   // all points. The order of label values must match that of label keys in the
   // metric descriptor.
-  repeated LabelValue label_values = 1;
+  LabelValues label_values = 1;
 
   // The data points of this timeseries.
   repeated DoubleValue points = 2;
@@ -134,7 +164,7 @@ message HistogramTimeSeries {
   // The set of label values that uniquely identify this timeseries. Applies to
   // all points. The order of label values must match that of label keys in the
   // metric descriptor.
-  repeated LabelValue label_values = 1;
+  LabelValues label_values = 1;
 
   // The data points of this timeseries.
   repeated HistogramValue points = 2;
@@ -146,20 +176,12 @@ message SummaryTimeSeries {
   // The set of label values that uniquely identify this timeseries. Applies to
   // all points. The order of label values must match that of label keys in the
   // metric descriptor.
-  repeated LabelValue label_values = 1;
+  LabelValues label_values = 1;
 
   // The data points of this timeseries.
   repeated SummaryValue points = 2;
 }
 
-message LabelValue {
-  // The value for the label.
-  string value = 1;
-  // If false the value field is ignored and considered not set.
-  // This is used to differentiate a missing label from an empty string.
-  bool has_value = 2;
-}
-
 // Int64Value is a timestamped measurement of int64 value.
 message Int64Value {
   // start_time_unixnano is the time when the cumulative value was reset to zero.