ts: include histogram quantiles in tsdump

[tbg]

cockroach debug tsdump previously silently did not return metrics
backed by histograms. This is for technical reasons related to the
bookkeeping of metrics names and is rectified here by requiring some
extra tagging of metrics that are histograms so that they can be picked
up by tsdump. It's not pretty, but pragmatic: it works and it'll be
clear to anyone adding a histogram in the future how to proceed, even if
they may wonder why things work in such a roundabout manner (and if
they're curious about that, the relevant issues are linked in comments
as well).

I also renamed AllMetricsNames to AllInternalTimeseriesMetricsNames
to make clear what is being returned.

Demo:

killall -9 cockroach; rm -rf cockroach-data;
./cockroach start-single-node --insecure --background && \
./cockroach workload run kv --init \
    'postgres://[email protected]:26257?sslmode=disable' --duration=300s && \
./cockroach debug tsdump --format=raw --insecure > tsdump.gob && \
killall -9 cockroach && rm -rf cockroach-data && \
COCKROACH_DEBUG_TS_IMPORT_FILE=tsdump.gob ./cockroach start-single-node --insecure

Release justification: low-risk observability fix
Release note (ops change): The ./cockroach debug tsdump command now
downloads histogram timeseries it silently omitted previously.

[cockroach-teamcity]

This change is 

[dhartunian]

thx for all the docs in the code. Helped me understand why things have to be done the way they were.

Reviewed 3 of 4 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @tbg)

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pkg/server/status_test.go, line 1049 at r1 (raw file):

				// AllInternalTimeseriesMetricNames(). It's not a complete check but good enough in
				// practice. Ideally we wouldn't require `histogramMetricsNames` and
				// the associated manual step when adding a histogram. See:

nit: is this comment incomplete?

[tbg]

TFTR! CI is red because of some LogicTest, so going to give it a shot

bors r=dhartunian

Dismissed @dhartunian from a discussion.
Reviewable status: complete! 0 of 0 LGTMs obtained (and 1 stale) (waiting on @dhartunian)

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pkg/server/status_test.go, line 1049 at r1 (raw file):

Previously, dhartunian (David Hartunian) wrote…

nit: is this comment incomplete?

Sure is! Fixed.

[craig]

This PR was included in a batch that was canceled, it will be automatically retried

[craig]

Build succeeded:

• GitHub CI (Cockroach)