refactor: add basic counter metrics for IMC

[Chethan-rao]

Type of Change

• Bugfix
• New feature
• Enhancement
• Refactoring
• Dependency updates
• Documentation
• CI/CD

Description

This PR introduces basic counter metrics for IMC -

• hits (successful read)
• misses (failed read)
• manual invalidations (done by applications) - makes use of eviction listener on moka that can help identify strategy based evictions

This PR also moves the add_attribute method of metrics to router_env crate. This method now takes a list of attributes as an argument as opposed to just one (key,value) pair.

Additional Changes

• This PR modifies the API contract
• This PR modifies the database schema
• This PR modifies application configuration/environment variables

Motivation and Context

How did you test it?

1. Insert new config key value pair. Data is not cached in IMC during insertion

curl --location 'http://localhost:8080/configs/' \
--header 'Content-Type: application/json' \
--header 'Accept: application/json' \
--header 'api-key: test_admin' \
--data '{
    "key": "test_key",
    "value": "test_val_1"
}'

2. Retrieve the inserted config so that the data gets cached to IMC. Since this is the first time the data is getting retrieved, there'll be a cache mis

curl --location 'http://localhost:8080/configs/test_key' \
--header 'Content-Type: application/json' \
--header 'Accept: application/json' \
--header 'api-key: test_admin' \
--data ''

3. Again retrieve the same config. Since in the previous retrieve, the data was cached, this retrieve will be a cache hit

curl --location 'http://localhost:8080/configs/test_key' \
--header 'Content-Type: application/json' \
--header 'Accept: application/json' \
--header 'api-key: test_admin' \
--data ''

4. Update the inserted config. This time the cache data has to be invalidated. This should increment the IN_MEMORY_CACHE_EVICTION_COUNT metric

curl --location 'http://localhost:8080/configs/test_key' \
--header 'Content-Type: application/json' \
--header 'Accept: application/json' \
--header 'api-key: test_admin' \
--data '{
    "key": "test_key",
    "value": "test_val_2"
}'

Checklist

• I formatted the code cargo +nightly fmt --all
• I addressed lints thrown by cargo clippy
• I reviewed the submitted code
• I added unit tests for my changes where possible