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