Atomically write the json file

[trazfr]

Hello,

I'm using your code and ended up with a lot of holes in the monitoring.
I also got many errors like this on Prometheus side (logs set to debug):

level=debug ts=2018-09-13T04:47:02.434227377Z caller=file.go:338 component="discovery manager scrape" discovery=file msg="file_sd refresh found file that should be removed" file=/opt/prometheus/etc/mdns-sd.json
level=error ts=2018-09-13T04:47:22.433789245Z caller=file.go:321 component="discovery manager scrape" discovery=file msg="Error reading file" path=/opt/prometheus/etc/mdns-sd.json err="unexpected end of JSON input"
level=debug ts=2018-09-13T04:47:22.434966794Z caller=file.go:338 component="discovery manager scrape" discovery=file msg="file_sd refresh found file that should be removed" file=/opt/prometheus/etc/mdns-sd.json

The issues seems that mdsn.go writes the file in a non atomic manner.
If prometheus reads the file when mdns.go writes it, the json may be truncated at the precise time prometheus reads it and fails to decode it.

Before:

• truncate the file with os.Create(). The file is still here but empty
• fill it
• flush and close it

Here I just used a lib to atomically write the new json file:

• write a new temporary file next to the old one
• flush and close it
• atomically rename it to replace the old one

System used (Raspberry Pi 3B):
prometheus-mdns compiled with:

• go1.10.3
• GOOS=linux
• GOARCH=arm
• GOARM=7

Prometheus used:

• https://github.com/prometheus/prometheus/releases/download/v2.4.0/prometheus-2.4.0.linux-armv7.tar.gz