Clarify supported compression methods for OTLP

Fixes: open-telemetry#1001 (comment)

Author: Tigran Najaryan

specification/protocol/exporter.md

@@ -13,9 +13,14 @@ The following configuration options MUST be available to configure the OTLP expo
 | 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     | 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              | The headers associated with gRPC or HTTP requests.                   | n/a               | `OTEL_EXPORTER_OTLP_HEADERS` `OTEL_EXPORTER_OTLP_SPAN_HEADERS` `OTEL_EXPORTER_OTLP_METRIC_HEADERS` |
-| Compression          | Compression key for supported compression types. Supported compression: `gzip`| no compression              | `OTEL_EXPORTER_OTLP_COMPRESSION` `OTEL_EXPORTER_OTLP_SPAN_COMPRESSION` `OTEL_EXPORTER_OTLP_METRIC_COMPRESSION` |
+| Compression          | Compression key for supported compression types. | No value              | `OTEL_EXPORTER_OTLP_COMPRESSION` `OTEL_EXPORTER_OTLP_SPAN_COMPRESSION` `OTEL_EXPORTER_OTLP_METRIC_COMPRESSION` |
 | Timeout              | Max waiting time for the backend to process each spans or metrics batch. | 10s               | `OTEL_EXPORTER_OTLP_TIMEOUT` `OTEL_EXPORTER_OTLP_SPAN_TIMEOUT` `OTEL_EXPORTER_OTLP_METRIC_TIMEOUT` |
 
+Supported values for OTEL_EXPORTER_OTLP_*COMPRESSION options:
+
+- If the value is missing then compressions is disabled.
+- `gzip` is the only specified compression method for now. Other options MAY be supported by language SDKs and should be documented for each particular language.
+
 Example 1
 
 The following configuration sends all signals to the same collector: