Added support for yaml-formatted configs

[marinafrank]

Description

• Added yaml parser for more readable configs. toml is also still supported.
• Enchanced log messages: use key-value syntax, added warning message when fallback to hard-coded default metrics
• Updated readme with some tips from my experience with the oracledb exporter (which I find anyway cool).

Type of change

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

How Has This Been Tested?

make local-build
export DATA_SOURCE_NAME="<connectionstring-to-oracle-19.21.0.0.0>"

# regression test
dist/oracledb_exporter-0.5.2.linux-amd64/oracledb_exporter --default.metrics=./default-metrics.toml --log.level debug
dist/oracledb_exporter-0.5.2.linux-amd64/oracledb_exporter --custom.metrics=./custom-metrics-example/custom-metrics.toml --log.level debug
curl http://localhost:9161

# functional test
dist/oracledb_exporter-0.5.2.linux-amd64/oracledb_exporter --default.metrics=./default-metrics.yaml --log.level debug
dist/oracledb_exporter-0.5.2.linux-amd64/oracledb_exporter --custom.metrics=./custom-metrics-example/custom-metrics.yaml --log.level debug
curl http://localhost:9161

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• Updated version in Makefile respecting semver v2

[Yannig]

Thanks for this contribution! Looks very interesting!

Do you mind to change just one thing to the YAML structure: use metrics for the root of all metrics?

Sorry for the delay ...

Yannig