[k6] Exclude timings when error occur

[k6] Exclude timings when error occur

If an error occurs during k6 test, e.g. tcp connection refused, the
timing of failed response is included in the average response time.
This can lead to incorrect plots when errors occur, e.g. operations
appear to be faster when really the operations are failing.

- use custom Trend metrics for all measurements
- only log results when operations return expected response codes

Issue-ID: CPS-2345
Signed-off-by: danielhanrahan <daniel.hanrahan@est.tech>
Change-Id: Iab1db5e5ca9530f33da239187348cabcd0605648