elastic-agent diagnostics collect --pprof can fail if the collected profile exceeds the gRPC message size limit

This was originally encountered in by a support engineer helping a user:

```
./elastic-agent diagnostics collect --pprof
Failed to gather pprof data from elastic-agent: rpc error: code = ResourceExhausted desc = grpc: received message larger than max (5855561 vs. 4194304)
Created diagnostics archive "elastic-agent-diagnostics-2022-07-13T07-08-08Z-00.zip"
***** WARNING *****
Created archive may contain plain text credentials.
Ensure that files in archive are redacted before sharing.
```

The default message size in gRPC in 4 MB which can be changed with https://pkg.go.dev/google.golang.org/grpc#MaxRecvMsgSize

A better solution is likely to chunk the profiles so they are guaranteed to stay under the configured message size limit.