[#4859] Change default value of flag metric_node_name to hostname:port

[vikramrajsitpal]

prometheus-metrics: change default metric_node_name to hostname:port

Default value for GFLAG metric_node_name was "DEFAULT_NODE_NAME". Changed this to hostname:web_server_port.
/prometheus-metrics now show new default value.

Summary:
Test (on localhost):
Command: ./bin/yb-ctl start --rf 1
On opening http://127.0.0.1:7000/prometheus-metrics, the exported_instance field shows hostname:webserverPort

Command with custom metric_node_name: ./bin/yb-ctl start --rf 1 --master_flags "metric_node_name=alpha:1"
On opening http://127.0.0.1:7000/prometheus-metrics, the exported_instance field shows alpha:1

[iSignal]

Looks good in general @vikramrajsitpal , I will run this through our build and unit test pipeline and get back to you.

[iSignal]

Can you please update the PR summary with a test plan? For ex: are change reflected in http://127.0.0.1:7000/prometheus-metrics when the flag is not specified vs explicitly specified?

[vikramrajsitpal]

Can you please update the PR summary with a test plan? For ex: are change reflected in http://127.0.0.1:7000/prometheus-metrics when the flag is not specified vs explicitly specified?

Done.

[iSignal]

Minor formatting comments. I will let you know what the results of the internal test pipeline are - just in case there are any unit tests that depend on this flag value.

[iSignal]

We should also need to set this flag by default for tablet servers too, so I would also add similar code to tablet_server_main.cc.

[vikramrajsitpal]

Changes added, tests performed for master and tablet server.

Command: ./bin/yb-ctl start --rf 2 --master_flags "metric_node_name=malpha:1" --tserver_flags "metric_node_name=talpha:88"

On opening http://127.0.0.1:9000/prometheus-metrics, the exported_instance field shows talpha:88.
Without the custom flag metric_node_name in --tserver_flags, the default value of hostname:9000 is shown.

[iSignal]

Looks good. I will run it through the internal tests once again to make sure nothing changes.

A couple of minor comments.

[iSignal]

Thanks for your patience @vikramrajsitpal ! Just one minor indentation issue remaining.

[vikramrajsitpal]

Had to force push as the commit message had a typo in the Issue number.

[iSignal]

There is a conflict in the CONTRIBUTORS.md file, apparently, could you rebase your branch to the latest commit in master, fix the conflict and possibly force push again?

[vikramrajsitpal]

There is a conflict in the CONTRIBUTORS.md file, apparently, could you rebase your branch to the latest commit in master, fix the conflict and possibly force push again?

So, I first merged my forked repo's master branch with yugabyte-db/master. Then I rebased my fix branch which is used in this PR and resolved the conflict. I have pushed my fix branch for now and it is visible here, but, it has an extra commit which it prompted me to make when I finished merging my master branch with ydb's master branch, so that is also visible. Please let me know if it is not acceptable.