Change "otlp/proto" reference to "otlp/protobuf" (open-telemetry#1032)

"proto" is a jargon, "protobuf" is the most commonly used abbreviation.

specification/protocol/exporter.md

@@ -9,7 +9,7 @@ The following configuration options MUST be available to configure the OTLP expo
 | Configuration Option | Description                                                  | Default           | Env variable                                                 |
 | -------------------- | ------------------------------------------------------------ | ----------------- | ------------------------------------------------------------ |
 | Endpoint             | Target to which the exporter is going to send spans or metrics. This MAY be configured to include a path (e.g. `example.com/v1/traces`). | `localhost:55680` | `OTEL_EXPORTER_OTLP_ENDPOINT` `OTEL_EXPORTER_OTLP_SPAN_ENDPOINT` `OTEL_EXPORTER_OTLP_METRIC_ENDPOINT` |
-| Protocol             | The protocol used to transmit the data. One of `grpc`,`http/json`,`http/proto`. | `grpc`               | `OTEL_EXPORTER_OTLP_PROTOCOL` `OTEL_EXPORTER_OTLP_SPAN_PROTOCOL` `OTEL_EXPORTER_OTLP_METRIC_PROTOCOL` |
+| Protocol             | The protocol used to transmit the data. One of `grpc`,`http/json`,`http/protobuf`. | `grpc`               | `OTEL_EXPORTER_OTLP_PROTOCOL` `OTEL_EXPORTER_OTLP_SPAN_PROTOCOL` `OTEL_EXPORTER_OTLP_METRIC_PROTOCOL` |
 | Insecure             | Whether to enable client transport security for the exporter's `grpc` or `http` connection. | `false`           | `OTEL_EXPORTER_OTLP_INSECURE` `OTEL_EXPORTER_OTLP_SPAN_INSECURE` `OTEL_EXPORTER_OTLP_METRIC_INSECURE` |
 | Certificate File     | Path to certificate file for TLS credentials of gRPC client. Should only be used if `insecure` is set to `false`. | n/a               | `OTEL_EXPORTER_OTLP_CERTIFICATE` `OTEL_EXPORTER_OTLP_SPAN_CERTIFICATE` `OTEL_EXPORTER_OTLP_METRIC_CERTIFICATE` |
 | Headers              | Key-value pairs to be used as headers associated with gRPC or HTTP requests. See [Specifying headers](./exporter.md#specifying-headers-via-environment-variables) for more details.                   | n/a               | `OTEL_EXPORTER_OTLP_HEADERS` `OTEL_EXPORTER_OTLP_SPAN_HEADERS` `OTEL_EXPORTER_OTLP_METRIC_HEADERS` |