clarify meaning of Certificate File (#1803)

* clarify meaning of "Certificate File"

The current version of the specification makes me think that Certificate File is intended to be used as part of the client authentication, which it is not. This change addresses part of the confusion discussed in open-telemetry/opentelemetry-specification#1363, the certificate file option and environment variable points to the certificate used to verify the server's certificate.

See also open-telemetry/opentelemetry-specification#1375.

* update changelog

* Update CHANGELOG.md

Co-authored-by: Sergey Kanzhelev <S.Kanzhelev@live.com>

specification/protocol/exporter.md

@@ -11,7 +11,7 @@ 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. The endpoint MUST be a valid URL with scheme (http or https) and host, and MAY contain a port and path. A scheme of https indicates a secure connection. When using `OTEL_EXPORTER_OTLP_ENDPOINT` with OTLP/HTTP, exporters SHOULD follow the collector convention of appending the version and signal to the path (e.g. `v1/traces` or `v1/metrics`). The per-signal endpoint configuration options take precedence and can be used to override this behavior. See the [OTLP Specification][otlphttp-req] for more details. | `https://localhost:4317` | `OTEL_EXPORTER_OTLP_ENDPOINT` `OTEL_EXPORTER_OTLP_TRACES_ENDPOINT` `OTEL_EXPORTER_OTLP_METRICS_ENDPOINT` |
-| Certificate File     | Path to certificate file for TLS credentials of gRPC client. Should only be used for a secure connection. | n/a               | `OTEL_EXPORTER_OTLP_CERTIFICATE` `OTEL_EXPORTER_OTLP_TRACES_CERTIFICATE` `OTEL_EXPORTER_OTLP_METRICS_CERTIFICATE` |
+| Certificate File     | The trusted certificate to use when verifying a server's TLS credentials. Should only be used for a secure connection. | n/a               | `OTEL_EXPORTER_OTLP_CERTIFICATE` `OTEL_EXPORTER_OTLP_TRACES_CERTIFICATE` `OTEL_EXPORTER_OTLP_METRICS_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_TRACES_HEADERS` `OTEL_EXPORTER_OTLP_METRICS_HEADERS` |
 | Compression          | Compression key for supported compression types. Supported compression: `gzip`| No value              | `OTEL_EXPORTER_OTLP_COMPRESSION` `OTEL_EXPORTER_OTLP_TRACES_COMPRESSION` `OTEL_EXPORTER_OTLP_METRICS_COMPRESSION` |
 | Timeout              | Maximum time the OTLP exporter will wait for each batch export. | 10s               | `OTEL_EXPORTER_OTLP_TIMEOUT` `OTEL_EXPORTER_OTLP_TRACES_TIMEOUT` `OTEL_EXPORTER_OTLP_METRICS_TIMEOUT` |