[Feat] Make it configurable to expose metrics endpoint

[yottahmd]

We would like to introduce a global configuration option to control whether the metrics API endpoint is publicly exposed.
To minimize unnecessary exposure, this endpoint will be disabled by default, and users can enable it as needed.

Config example:

server:
  metrics: public # default: private

Environment env: DAGU_SERVER_METRICS=public

Any suggestions or concerns are very welcome.
cc: @jeremydelattre59 @ghansham

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Suggested by @kriyanshii

server:
  metricsType: 

[kriyanshii]

for starters it should have two types of metrics - brief and debrief.

1. Shares the no of successful, failed, canceled number.
2. can share dagwise dag-run stats.