OpenTelemetry-Go: Log gRPC exporter ignores env TLS certs, bypassing mTLS/pinning

### Summary The OTLP log gRPC exporter loads TLS settings from environment variables but does not apply them when creating gRPC transport credentials. Operators who rely on `OTEL_EXPORTER_OTLP_LOGS_CERTIFICATE`, `OTEL_EXPORTER_OTLP_CERTIFICATE`, or related client certificate variables for CA pinning or mTLS get a connection that falls back to system roots and omits the env-supplied client certificate. A network attacker who can intercept or spoof the collector connection with a system-trusted certificate can read or alter log telemetry. Introduced in commit: d99c76f ### Details The affected code is in `exporters/otlp/otlplog/otlploggrpc`. `newConfig` resolves env-based TLS configuration into `cfg.tlsCfg` at `exporters/otlp/otlplog/otlploggrpc/config.go:106-116`. The finding also identifies `loadEnvTLS` at `config.go:451-492` as the code that builds a `*tls.Config` containing `RootCAs` and client certificates from `OTEL_EXPORTER_OTLP[_LOGS]_CERTIFICATE` and `OTEL_EXPORTER_OTLP[_LOGS]_CLIENT_CERTIFICATE`/`KEY`. However, `newGRPCDialOptions` in `exporters/otlp/otlplog/otlploggrpc/client.go:83-92` only checks `cfg.gRPCCredentials` and `cfg.insecure`. When neither is set, which is the normal env-only TLS configuration path, it uses `credentials.NewTLS(nil)`. That default trusts the host system root CAs and contains no env-supplied client certificate. The finding evidence reports no other `tlsCfg` use in the package, so env-based CA pinning and mTLS settings are loaded but not enfo