Prepare v0.12.3 release 

core: avoid collecting empty metric families 

Merge pull request jupp0r#453 from jupp0r/fix-std-tochars core: Fix usage of std::to_chars

Merge pull request jupp0r#435 from jupp0r/improve-example pull: Add better example

pull: Use internal base64 decoding function to avoid dependency (jupp… …0r#417)

chore: Raise version to v0.10.0 due to changed public interface 

chore: Raise version to v0.10.0 due to changed public interface 

Include missing <ostream> 

Merge pull request jupp0r#310 from jupp0r/update-badges chore(doc): Add GitHub CI badge

Merge pull request jupp0r#273 from jupp0r/propagate-curl-errors Fix push gateway code