feat: Add a metric to measure the tile count of adm response

[ncloudioj]

This closes DISCO-2346.

r? @pjenvey for extra eyeballs since this is the first time to use a distribution metric in Contile.

[ncloudioj]

We could either increment a counter such as contile.tiles.adm.response with a new tag tile.count, but that will add this tag to all the subsequent metrics which would be wasteful, or use a plain counter contile.tiles.adm.response.tile_count, but that'll be not super easy to use for monitoring & analysis.

[pjenvey]

@ncloudioj FYI the distribution type isn't supported by telegraf's default configuration, however it can parse them when both these are set: datadog_extensions = true (which we already have) and datadog_distributions = true (which we don't).

So this will require to SRE add that.

[pjenvey]

I don't care if 'distribution' is included in this name but maybe something more specific:

Suggested change

	"tiles.adm.response.distribution",
	"tiles.adm.response.tiles_count",

[ncloudioj]

Thanks, Phil! I will double check with SRE to see if those changes are viable. If not, will reformat this to other metric types.

[ncloudioj]

Per SRE, we're using a pretty old version of InfluxDB so its support of the distribution metric type is unclear. Also, this work is due on May 5th, so let's use a counter metric for this for now.

[pjenvey]

Count obviously isn't ideal here obviously but I guess we can at least look at the breakdown of counts over small intervals tagged by e.g. geo country code.