feat: add hugepages into the HostMemoryFull alert expression

[lucabello]

Issue

Closes #41.

Solution

Warning

I had to bump the cos_agent library because of a linting error which was auto-fixed by tox -e fmt.

Modify the alert expression as detailed in the issue. The expression was flipped because there is a MemFree_bytes metric, but not a MemUsed_bytes one.

TL;DR (see comments in the yaml below):

expr: |
      100 * avg_over_time(node_memory_MemFree_bytes[1m]) /
        (
          avg_over_time(node_memory_MemTotal_bytes[1m])
          - avg_over_time(node_memory_Hugetlb_bytes[1m])
        ) < 10

Here's a visual representation:

🔴 Pre-allocated HugePages memory (node_memory_Hugetlb_bytes)
🟣 Memory used outside of HugePages (node_memory_MemTotal_bytes - node_memory_Hugetlb_bytes)
🔵 Available memory (node_memory_MemFree_bytes)

🔴 + 🟣 = node_memory_MemTotal_bytes

Memory: 🔴🔴🔴🔴🔴🔴🔴🟣🟣🟣🟣🔵🔵

The alert is checking that 🔵/🟣 is less than 10%.