Clean up rrdp_metrics if the value has not been changed for some time

[ties]

Some repositories use dynamic hostnames. For example fraunhofer.de.

Let's prevent this situation:

rpkiclient_fetch_status_total{type="rrdp_tls_certificate_verification_failed",uri="https://13ab95dc-d1ba-4a22-beae-a8c4034a18a5.rpki.rpki-test.sit.fraunhofer.d..."} 1.0
rpkiclient_fetch_status_total{type="rrdp_tls_certificate_verification_failed",uri="https://4b6081b9-92ca-482e-aa09-cc5d4c69d567.rpki.rpki-test.sit.fraunhofer.d..."} 1.0
rpkiclient_fetch_status_total{type="rrdp_tls_certificate_verification_failed",uri="https://3310d868-8ba9-4eef-90f6-5ed67b8dfca6.rpki.rpki-test.sit.fraunhofer.d..."} 1.0
rpkiclient_fetch_status_total{type="rrdp_tls_certificate_verification_failed",uri="https://e0797c81-b9d3-455f-a137-e170f80e223d.rpki.rpki-test.sit.fraunhofer.d..."} 1.0
rpkiclient_fetch_status_total{type="rrdp_tls_certificate_verification_failed",uri="https://ec123af1-7564-4ae1-8a09-b84072120631.rpki.rpki-test.sit.fraunhofer.d..."} 1.0
rpkiclient_fetch_status_total{type="rrdp_tls_certificate_verification_failed",uri="https://8f26fb0a-5089-464f-b4bc-7d6ebd290d8f.rpki.rpki-test.sit.fraunhofer.d..."} 1.0
rpkiclient_fetch_status_total{type="rrdp_tls_certificate_verification_failed",uri="https://bc1d0e6c-f606-4189-a7ef-5ad50098fca4.rpki.rpki-test.sit.fraunhofer.d..."} 1.0
rpkiclient_fetch_status_total{type="rrdp_tls_certificate_verification_failed",uri="https://8ae9d969-93e0-45a9-97b0-d8eda5b5fbde.rpki.rpki-test.sit.fraunhofer.d..."} 1.0
rpkiclient_fetch_status_total{type="rrdp_tls_certificate_verification_failed",uri="https://a5a8f620-5919-424e-b3b6-1561bae94490.rpki.rpki-test.sit.fraunhofer.d..."} 1.0
rpkiclient_fetch_status_total{type="rrdp_tls_certificate_verification_failed",uri="https://992efabb-4ad6-4055-b97b-9d27449258a7.rpki.rpki-test.sit.fraunhofer.d..."} 1.0
rpkiclient_fetch_status_total{type="rrdp_tls_certificate_verification_failed",uri="https://59397f45-e5b0-4e46-acb6-c91f50f2696a.rpki.rpki-test.sit.fraunhofer.d..."} 1.0
rpkiclient_fetch_status_total{type="rrdp_tls_certificate_verification_failed",uri="https://4a379aa9-3f2e-4631-9eda-9d20d835dcda.rpki.rpki-test.sit.fraunhofer.d..."} 1.0
rpkiclient_fetch_status_total{type="rrdp_tls_certificate_verification_failed",uri="https://8821de42-4562-4e3a-996a-f2c84f3b865b.rpki.rpki-test.sit.fraunhofer.d..."} 1.0
rpkiclient_fetch_status_total{type="rrdp_tls_certificate_verification_failed",uri="https://68a54795-8bb5-46aa-999d-84992770de8d.rpki.rpki-test.sit.fraunhofer.d..."} 1.0