Parameters for private key and its chain added (#2370)

* Parameters for private key and its chain added

* CLIENT_KEY_CHAIN renamed to CLIENT_CERTIFICATE

specification/protocol/exporter.md

@@ -24,6 +24,14 @@ The following configuration options MUST be available to configure the OTLP expo
   - Default: n/a
   - Env vars: `OTEL_EXPORTER_OTLP_CERTIFICATE` `OTEL_EXPORTER_OTLP_TRACES_CERTIFICATE` `OTEL_EXPORTER_OTLP_METRICS_CERTIFICATE`
 
+- **Client key file**: Clients private key to use in mTLS communication in PEM format.
+  - Default: n/a
+  - Env vars: `OTEL_EXPORTER_OTLP_CLIENT_KEY` `OTEL_EXPORTER_OTLP_TRACES_CLIENT_KEY` `OTEL_EXPORTER_OTLP_METRICS_CLIENT_KEY`
+
+- **Client certificate file**: Client certificate/chain trust for clients private key to use in mTLS communication in PEM format.
+  - Default: n/a
+  - Env vars: `OTEL_EXPORTER_OTLP_CLIENT_CERTIFICATE` `OTEL_EXPORTER_OTLP_TRACES_CLIENT_CERTIFICATE` `OTEL_EXPORTER_OTLP_METRICS_CLIENT_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.
   - Default: n/a
   - Env vars: `OTEL_EXPORTER_OTLP_HEADERS` `OTEL_EXPORTER_OTLP_TRACES_HEADERS` `OTEL_EXPORTER_OTLP_METRICS_HEADERS`