request help: how should i get the custom header (this header was added beforehand in an ext-plugin-pre-req plugin.filter method) in ext-plugin-post-resp plugin???

#### Issue description
I added an request header (startTimestamp) in an ext-plugin-pre-req plugin using HttpRequest.setHeader() method, then I want to acquire this custom header in an ext-plugin-post-resp plugin using PostRequest.getUpstreamHeaders() method. But it doesn't work. I've tried serveral ways, and none of them worked. This problem bothered me a lot, could you help me? Really thanks.

### Environment
* apisix-java-plugin-runner version
0.3.0
* apisix verison
2.15.1