Change trace_sample field to bool

Moved away from traceContext regex

use correct field for "labels"

This was implemented in StackDriver in
GoogleCloudPlatform/fluent-plugin-google-cloud#292.

fixes #4

fix tests for go modules setup

Merge pull request #14 from ownersroom/fix/mutex-unlocking

Fix panic due to missing mutex lock/unlock

Merge pull request #12 from blendle/mutex-2

add missing mutex locks

Merge pull request #11 from blendle/mutex

Add mutex to protect against race conditions

Merge pull request #8 from dstotijn/http-req-typo-fix

Fix JSON key typos for `httpRequest` log object

Merge pull request #6 from blendle/time-key

Use correct timestamp key name

Merge pull request #5 from blendle/nanoseconds-ts

Use the correct format for the message timestamp