server: split the tenant server creation into 3 stages

[knz]

Needed as prerequisite to #90176, towards fixing #89974.
First two commits from #90523.

The original motivation (and ultimate goal) for this PR is to
split the tenant server initialization into three phases: New(),
PreStart(), AcceptClients(), so as to reuse a common process startup
logic in a separate PR (#90176).

To achieve this goal, this PR re-orders the initialization steps
in server.NewTenantServer (previously known as
server.StartTenant), and extracts many of them into a new method
(*SQLServerWrapper).PreStart().

The specific order of the code in NewTenantServer() and
(*SQLServerWrapper).PreStart() was chosen to mirror the order of
things in NewServer() and (*Server).PreStart().

Reason for using the same order:

• it makes the review of this change easier: the reviewer can pull
  server.go and tenant.go and read them side-by-side, to satisfy
  themselves that the two implementations of
  NewServer/NewTenantServer and PreStart are equivalent.

• it will make it easier for future maintainers to keep them in sync.

• it helps us discover the fact that both sides share a lot of code.
  This opens an opportunity to merge them to a common implementation
  at a later stage.

While doing this work, care was also taken to discover which steps of
(*Server).PreStart() were missing from the tenant server
initialization. We found the following:

• the Sentry context enhancement (to report cluster ID, etc)
  was missing. This commit fixes that.

• several log entries that report the server configuration
  to the OPS channel were not emitted. This commit fixes that.

• the Graphite metric reporting was never enabled, even when
  configured. This commit fixes that.

• the Obs Service testing knobs (TestingKnobs.EventExporter) were
  not configured on the ObsServer instance. This commit fixes that.

• the go.scheduler_latency metric was not being measured.
  This commit fixes that.

Additionally, two followup issues were filed for the following
missing steps:

• missing support for the special file that blocks background jobs. (server: missing support for DISABLE_STARTING_BACKGROUND_JOBS in secondary tenants #90524)
• missing support for the system.eventlog cleanup loop. (server: old data in system.eventlog is not deleted for secondary tenants #90521)

Epic: CRDB-14537

[cockroach-teamcity]

This change is 

[knz]

@jeffswenson @dhartunian I'd like to invite you both to review this, with different focus:

• @jeffswenson to satisfy yourself that the changes here are feature-neutral, despite the relative large code movement. (Hint: look at the suggestion in the PR description to place the source files side-by-side to make review easier.)
• @dhartunian because we have discussed this area further, and I know you have an interest in keeping your understanding of this area up-to-date.

cc @andreimatei.

[knz]

TFYR

bors r=dhartunian

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)