stats: export vttablet_tablet_type to prometheus

[maxenglander]

Description

This PR is very similar in spirit to #12772.

We already have a TabletType string that is exported to Golang expvars. E.g. after ./examples/local/101_initial_cluster.sh:

bash-5.2$ curl -s localhost:15100/debug/vars | jq -r '.TabletType'
primary

However, this is not exported to Prometheus. It's a pretty valuable thing to know for monitoring purposes, e.g. to detect and alert on conditions like "primary tablet is not serving".

This PR exposes the vttablet_tablet_type via /metrics, without changing the shape of the data currently exported via /debug/vars.

E.g., after ./examples/local/101_initial_cluster.sh.

bash-5.2$ curl -s localhost:15100/metrics | grep tablet_type{
vttablet_tablet_type{tablet_type="primary"} 1
bash-5.2$ curl -s localhost:15101/metrics | grep tablet_type{
vttablet_tablet_type{tablet_type="replica"} 1
bash-5.2$ curl -s localhost:15102/metrics | grep tablet_type{
vttablet_tablet_type{tablet_type="rdonly"} 1

Notes

tablet_type label

A note about the label name, tablet_type. There is another metric vttablet_tablet_type_count that has the label type, but there is another set of more recent metrics by @rafer #13521 that have the label tablet_type. I had to pick one of these, so I went with the more recent one, which I happen to prefer, personally.

The "right way"

The "right way" to export Prometheus metrics is to define the time series at process start up, and then only change their value.

That means that this metric in its current form is using Prometheus the "wrong way" - a ChangeTabletType or reparent will change the value of the tablet_type label, which from Prometheus' point of view means that one time series has disappeared, and a new time series has suddenly appeared.

The current way should be somewhat usable, but will prevent Prometheus users from using the metric to its fullest.

Perhaps a better approach would be to define a new metric like vttablet_tablet_types, which is a map of tablet types to either 0 or 1. These time series would be registered at VTTablet startup, and then change during ChangeTabletType and reparents. I opted not to do this because it seemed like a bigger change, and would also emit a lot more metrics. But happy to change it if people prefer.

Related Issue(s)

Fixes #14300

Checklist

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[maxenglander]

I don't see a page in docs where vttablet metrics are documented.

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