Misc improvements

[LotharSee]

• use a different metric tag to count traces manually prioritized
• fix multiline logging
• expire receiverStats tagset

Example of previous output: multi-line, one tag set.

2017-10-12 12:19:39 INFO (receiver.go:316) -
        [lang:go lang_version:1.9.1 interpreter:gc-amd64-linux tracer_version:v0.5.0] -> traces received: 21358, traces dropped: 0, traces filtered: 0, traces amount: 14167935 bytes, services received: 0, services amount: 0 bytes
        [] -> traces received: 0, traces dropped: 0, traces filtered: 0, traces amount: 0 bytes, services received: 0, services amount: 0 bytes

After this PR, is should simply look like:

2017-10-12 12:19:39 INFO (receiver.go:316) - [lang:go lang_version:1.9.1 interpreter:gc-amd64-linux tracer_version:v0.5.0] -> traces received: 21358, traces dropped: 0, traces filtered: 0, traces amount: 14167935 bytes, services received: 0, services amount: 0 bytes

[ufoot]

Good for me.

[ufoot]

Nice catch, indeed, should look cleaner with this.