Expose coin price metrics

[isegall-da]

Fixes https://github.com/DACH-NY/canton-network-internal/issues/2181

Will add a dashboard and alerts in a separate PR, but I don't consider that a release blocker so didn't want to delay this PR

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[martinflorian-da]

Looks right to me, thanks a lot!

Did you sanity check on a cluster grafana that the metrics look as expected? Might be a good TODO before cutting the release, in case we did miss something here.

[isegall-da]

Did you sanity check on a cluster grafana that the metrics look as expected? Might be a good TODO before cutting the release, in case we did miss something here.

Yes, I did. Didn't trust myself enough to even write the test before looking at Grafana for sanity ;)