protect against subscriber file corruption

[kruskall]

APM Server version (apm-server version): 8.16

Description of the problem including expected versus actual behavior:

We're writing the subscriber position using os.WriteFile at

apm-server/x-pack/apm-server/sampling/processor.go

Line 608 in 840fe6c

return os.WriteFile(filepath.Join(storageDir, subscriberPositionFile), data, 0644)

The documentation says:

// Since WriteFile requires multiple system calls to complete, a failure mid-operation
// can leave the file in a partially written state.

We should ensure we're not leaving behind a corrupted file if write returns an error. A malformed/corrupted subscriber file could prevent APM Server from starting

Steps to reproduce:

1. create a corrupted subscriber file
2. start apm server

Provide logs (if relevant):

We've observed this a few times with the error

error parsing subscriber position file: unexpected end of JSON input