Adds database connection object with DTrace probes

[bnaecker]

• Uses the diesel_dtrace::DTraceConnection object for connecting to
  CockroachDB, which contains probes that fire when a connection is made
  and for each query.
• Adds a Nexus configuration option for handling the failures of probe
  registration. If true the application aborts if the probes cannot be
  registered on startup, otherwise a log message is printed.

[bnaecker]

For reference, here's the output of the query probes when nexus and oximeter start:

bnaecker@shale : ~/omicron $ pfexec dtrace -x strsize=4k -Zqn 'diesel_db*:::query_start { printf("%d: %s\n", arg0, copyinstr(arg1)); }'
115964116993: SELECT "saga"."id", "saga"."creator", "saga"."template_name", "saga"."time_created", "saga"."saga_params", "saga"."saga_state", "saga"."current_sec", "saga"."adopt_generation", "saga"."adopt_time" FROM "saga" WHERE (("saga"."saga_state" != $1) AND ("saga"."current_sec" = $2)) ORDER BY "saga"."id" ASC  LIMIT $3 -- binds: ["done", e6bff1ff-24fb-49dc-a54e-c6a350cd4d6c, 100]
115964116994: INSERT INTO "oximeter" ("id", "time_created", "time_modified", "ip", "port") VALUES ($1, $2, $3, $4, $5) ON CONFLICT ("id") DO UPDATE SET "time_modified" = $6, "ip" = $7, "port" = $8 -- binds: [1da65e5b-210c-4859-a7d7-200c1e659972, 2021-12-07T01:50:40.935912638Z, 2021-12-07T01:50:40.935912638Z, V6(Ipv6Network { addr: ::1, prefix: 128 }), 12223, 2021-12-07T01:50:40.936000269Z, V6(Ipv6Network { addr: ::1, prefix: 128 }), 12223]
115964116995: SELECT "oximeter"."id", "oximeter"."time_created", "oximeter"."time_modified", "oximeter"."ip", "oximeter"."port" FROM "oximeter" ORDER BY "oximeter"."id" ASC  LIMIT $1 -- binds: [1]
115964116996: SELECT "saga"."id", "saga"."creator", "saga"."template_name", "saga"."time_created", "saga"."saga_params", "saga"."saga_state", "saga"."current_sec", "saga"."adopt_generation", "saga"."adopt_time" FROM "saga" WHERE (("saga"."saga_state" != $1) AND ("saga"."current_sec" = $2)) ORDER BY "saga"."id" ASC  LIMIT $3 -- binds: ["done", e6bff1ff-24fb-49dc-a54e-c6a350cd4d6c, 100]
115964116997: INSERT INTO "oximeter" ("id", "time_created", "time_modified", "ip", "port") VALUES ($1, $2, $3, $4, $5) ON CONFLICT ("id") DO UPDATE SET "time_modified" = $6, "ip" = $7, "port" = $8 -- binds: [1da65e5b-210c-4859-a7d7-200c1e659972, 2021-12-07T01:50:57.035735022Z, 2021-12-07T01:50:57.035735022Z, V6(Ipv6Network { addr: ::1, prefix: 128 }), 12223, 2021-12-07T01:50:57.035753310Z, V6(Ipv6Network { addr: ::1, prefix: 128 }), 12223]
115964116998: SELECT "metric_producer"."id", "metric_producer"."time_created", "metric_producer"."time_modified", "metric_producer"."ip", "metric_producer"."port", "metric_producer"."interval", "metric_producer"."base_route", "metric_producer"."oximeter_id" FROM "metric_producer" WHERE ("metric_producer"."oximeter_id" = $1) ORDER BY "metric_producer"."oximeter_id", "metric_producer"."id" LIMIT $2 -- binds: [1da65e5b-210c-4859-a7d7-200c1e659972, 100]
115964116999: SELECT "oximeter"."id", "oximeter"."time_created", "oximeter"."time_modified", "oximeter"."ip", "oximeter"."port" FROM "oximeter" ORDER BY "oximeter"."id" ASC  LIMIT $1 -- binds: [1]
115964117000: INSERT INTO "metric_producer" ("id", "time_created", "time_modified", "ip", "port", "interval", "base_route", "oximeter_id") VALUES ($1, $2, $3, $4, $5, $6, $7, $8) ON CONFLICT ("id") DO UPDATE SET "time_modified" = $9, "ip" = $10, "port" = $11, "interval" = $12, "base_route" = $13 -- binds: [e6bff1ff-24fb-49dc-a54e-c6a350cd4d6c, 2021-12-07T01:50:57.065900961Z, 2021-12-07T01:50:57.065900961Z, V4(Ipv4Network { addr: 127.0.0.1, prefix: 32 }), 12221, 10.0, "/metrics/collect", 1da65e5b-210c-4859-a7d7-200c1e659972, 2021-12-07T01:50:57.066000104Z, V4(Ipv4Network { addr: 127.0.0.1, prefix: 32 }), 12221, 10.0, "/metrics/collect"]
115964117001: SELECT "saga"."id", "saga"."creator", "saga"."template_name", "saga"."time_created", "saga"."saga_params", "saga"."saga_state", "saga"."current_sec", "saga"."adopt_generation", "saga"."adopt_time" FROM "saga" WHERE (("saga"."saga_state" != $1) AND ("saga"."current_sec" = $2)) ORDER BY "saga"."id" ASC  LIMIT $3 -- binds: ["done", e6bff1ff-24fb-49dc-a54e-c6a350cd4d6c, 100]
115964117002: SELECT "saga"."id", "saga"."creator", "saga"."template_name", "saga"."time_created", "saga"."saga_params", "saga"."saga_state", "saga"."current_sec", "saga"."adopt_generation", "saga"."adopt_time" FROM "saga" WHERE (("saga"."saga_state" != $1) AND ("saga"."current_sec" = $2)) ORDER BY "saga"."id" ASC  LIMIT $3 -- binds: ["done", e6bff1ff-24fb-49dc-a54e-c6a350cd4d6c, 100]

The first argument is a unique identifier that allows correlating the start/end probes for both connections and queries.

[smklein]

Looks great, thank you for driving this!