add enabled configuration for metrics

[zhengyangyong]

Signed-off-by: zhengyangyong yangyong.zheng@qq.com

Ⅰ. Describe what this PR did

add a new configuration enabled for metrics:

## metrics settings
metrics {
  enabled = true                                          <-------- new
  registry-type = "compact"
  # multi exporters use comma divided
  exporter-list = "prometheus"
  exporter-prometheus-port = 9898
}

Ⅱ. Does this pull request fix one issue?

Ⅲ. Why don't you add test cases (unit test/integration test)?

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

[codecov-io]

Codecov Report

Merging #1266 into develop will decrease coverage by 0.03%.
The diff coverage is 62.5%.

@@              Coverage Diff              @@
##             develop    #1266      +/-   ##
=============================================
- Coverage      49.47%   49.43%   -0.04%     
  Complexity      1651     1651              
=============================================
  Files            326      326              
  Lines          11439    11441       +2     
  Branches        1415     1416       +1     
=============================================
- Hits            5659     5656       -3     
- Misses          5153     5155       +2     
- Partials         627      630       +3

Impacted Files	Coverage Δ	Complexity Δ
...ava/io/seata/core/constants/ConfigurationKeys.java	0% <ø> (ø)	0 <0> (ø)	⬇️
...n/java/io/seata/server/metrics/MetricsManager.java	76.92% <62.5%> (-4.9%)	5 <2> (ø)
...server/store/file/FileTransactionStoreManager.java	45.99% <0%> (-1.4%)	19% <0%> (ø)

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[slievrly]

LGTM

[CoffeeLatte007]

LGTM