Added debug logs for series lookup issues

[jkohen]

Debug logs will be added whenever the prometheus_sidecar/dropped_series metric is incremented. This also increases test coverage of the error paths.

This should help users debug issues caused by incorrect configuration, including:

• Removal or modification of the job and instance labels.
• Unsupported service discovery.
• Missing metric type metadata.

These are debug level instead of warning, because they will be logged for each failing series every time they are refreshed (every 3 minutes by default).

Closes #109
/cc @Dnefedkin

[StevenYCChou]

Request change on error handling ordering.

the initialization shortening is optional, but I think it will reduce line # and hence the time to consume the code.