Ignore samples with different values with same timestamp on ingest

[proddata]

Summary of the changes / Why this is an improvement

Considering that Prometheus doesn't allow samples with same timestamp and different values and enforcing this more strictly with newer version, just adding a ON CONFLICT DO NOTHING should be sufficient.

Checklist

• Link to issue this PR refers to (if applicable): Handle primary key conflicts better (upserts or ignore) #135
• CLA is signed

[amotl]

Thanks a stack for this!

[matriv]

thx @proddata !
Makes sense to resolve the issue like this, since we don't have the info to choose if we overwrite or not.