refactor(dashboard): refactor back pressure api path name

[yufansong]

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What's changed and what's your intention?

As suggest in #14863 comments, do some refactor to seperate the embeded back pressure and prometheus back pressure.
Change /metrics/actor/back_pressures to /metrics/fragment/prometheus_back_pressures
Change /metrics/back_pressures to /metrics/fragment/embedded_back_pressures

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[fuyufjh]

This will actually break the old-version RW, but it's acceptable to me.

This is because RisingWave's Web resources are served from GitHub repo dashboard-artifact branch, and it's not versioned, i.e. only the latest main build will be kept.

risingwave/docker/Dockerfile.hdfs

Lines 30 to 31 in ae7d7ba

	RUN curl -LO https://github.com/risingwavelabs/risingwave/archive/refs/heads/dashboard-artifact.zip
	RUN unzip dashboard-artifact.zip && mv risingwave-dashboard-artifact /risingwave/ui && rm dashboard-artifact.zip

Hope someday we can ship the frontend resource into / along with the binary.