module/apmgrpc: traces are broken by sampling

[axw]

Describe the bug

As seen in https://discuss.elastic.co/t/only-common-trace-id-for-sampled-spans/192459, sampling is breaking distributed tracing in apmgrpc.

Expected behavior

The apmgrpc client interceptor should include the transaction's trace context when spans are dropped (e.g. due to sampling or the max spans limit has been reached), like in the apmhttp client instrumentation.