metrics.proto will cause compilation errors when the protobuf version is less than 3.15.0

[ninghejun]

When I compile with protobuf v3.8.0, some compile errors occurred.
The compilation errors is as follows：

opentelemetry/proto/metrics/v1/metrics.proto:417:12: Explicit 'optional' labels are disallowed in the Proto3 syntax. To define 'optional' fields in Proto3, simply remove the 'optional' label, as fields are 'optional' by default.
opentelemetry/proto/metrics/v1/metrics.proto:452:12: Explicit 'optional' labels are disallowed in the Proto3 syntax. To define 'optional' fields in Proto3, simply remove the 'optional' label, as fields are 'optional' by default.
......
opentelemetry/proto/collector/metrics/v1/metrics_service.proto:19:1: Import "opentelemetry/proto/metrics/v1/metrics.proto" was not found or had errors.
opentelemetry/proto/collector/metrics/v1/metrics_service.proto:42:12: "opentelemetry.proto.metrics.v1.ResourceMetrics" is not defined.

Then I found that versions below protobuf v3.15.0 do not support optional labels. This problem needs to be fixed because protobuf v3.8.0 is heavily used in our business and it is difficult to upgrade. This prevents us from using opentelemetry.