fix bugs on configuration pages and refactor code

[ansjcy]

Description

Currently when we fetch the settings from backend, there are 2 defects:

• We are not considering any transient settings
• When obtaining window size / top n sizes from settings, if a metric is enabled but no default settings are set, we are setting "undefined" value to a state, which makes the setState function fail - In this case, if there are values in the settings for other metrics, the old values will be used.

This PR resolved both of the issues by assigning default values when reading settings data from backend. Also make sure we pick up the transient settings as well

Issues Resolved

#31
#29

Results

• Run a clean OpenSearch Cluster
• Start OSD with query insights plugin
• insert configurations

curl -X PUT 'localhost:9200/_cluster/settings' -H 'Content-Type: application/json' -d'
{
    "persistent" : {
         "search.insights.top_queries.latency.enabled" : true,
        "search.insights.top_queries.cpu.enabled" : true,
        "search.insights.top_queries.memory.enabled" : true,
        "search.insights.top_queries.group_by" : "none",
        "search.insights.top_queries.latency.window_size" : "1m",
        "search.insights.top_queries.latency.top_n_size" : 5
    }
}'

• check configuration page, the latency setting is correctly picked up

• Check CPU setting, the default values matches what's on the backend

- Check Memory setting, the default values matches what's on the backend

• Override with transient settings to disable top queries by latency

curl -X PUT 'localhost:9200/_cluster/settings' -H 'Content-Type: application/json' -d'
{
    "transient" : {
        "search.insights.top_queries.latency.enabled" : "false"
    }
}'

• Refresh the page and the transient settings are picked up

• Override with transient settings to enable grouping

curl -X PUT 'localhost:9200/_cluster/settings' -H 'Content-Type: application/json' -d'
{
    "transient" : {
        "search.insights.top_queries.group_by" : "similarity"
    }
}'

• Refresh the page and the transient settings are picked up for group by

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[deshsidd]

Looks good overall and definitely a more elegant way to solve the 2 bugs. We should add some tests to make sure we don't regress in the future but can do it as a follow-up due to the tight timelines?

[deshsidd]

nit: can name this to make the code more readable.