Add dynamic gathering of metrics based on Ceph schema socket output

[mkleina]

Please don't merge yet, it's a fully working concept, but needs to be accepted and finished (documentation updates and post-review fixes).

Fixes panics when metric namespace is incomplete. Fixes problems with unavailable metrics on some Ceph setups (fixes #26) Fixes #9 - now all /metric/avgcount and /metric/sum metrics are collapsed into /metric (with value of /sum divided by /avgcount).

Summary of changes:

• Refactored code with support for dynamic metric catalog generation based on Ceph socket data (both namespaces and descriptions are gathered from Ceph)
• Added support for delta metrics
• Added dynamic metric unit detection and metric data type (float64 or uint64) based on unit
• Updated unit tests to support dynamic metric gathering and delta metrics
• Updated plugin and unit tests to support snap-plugin-lib-go
• Removed fixed metric list documentation files and replaced with single METRICS.md (complete metric list for all daemons with latest Ceph version)

How to verify it:

• Load plugin
• Create global config with ceph socket path and binary path provided (global config is needed for GetMetricTypes to build metric catalog dynamically)
• Create task for collection of /intel/storage/ceph/*
• Try to call snaptel metric list, complete metric list should be available
• Try to call snaptel task watch to get values for all available Ceph metrics.

Testing done:

• Manual testing
• Unit tests (small)

[katarzyna-z]

Please squash commits before merging.

[PatrykMatyjasek]

Please squash commits

[mkleina]

@PatrykMatyjasek Done :+1:

[PatrykMatyjasek]

LGTM