PrometheusGraph component time display is not friendliness

[zhukai1991]

The TimestampTag component cannot customize the time format, and UTC is not a good display time format

[fbarl]

@zk923852367 Thanks for opening this issue!

UTC time format is the one we're using across our product Weave Cloud. While there is a side discussion going on about supporting local user time zone and indeed custom time formats in our app, the current situation is that we only need TimestampTag component to display times in UTC.

Having said that, feel free to open a PR that makes the component accept a custom time zone prop to make it more robust if that's how you wanted to use it for your purposes - it will probably come in handy for us as well at some point ;)