gagent tracing receivers adr (#190)

decision-records/2024-07-05--tempo-receivers.md

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+**Date:** 2024-06-03<br/>
+**Author:** Pietro Pasotti (@ppasotti)  
+
+
+## Decision 
+
+- Tempo-k8s should support all receivers that upstream Tempo supports
+- Grafana-agent should support all receivers that Tempo-k8s supports
+- For accepting traces from uncharmed applications:
+  - Tempo-k8s has one "force-enable this config option" per protocol
+  - Grafana-agent(vm AND -k8s) has one "force-enable this config option" per protocol
+- Grafana-agent proxies traces to Tempo exclusively via otlp_grpc (we cannot change this), and sends charm traces via otlp_http
+- Grafana-agent can be instructed to enable a specific (set of) protocol through cos-agent relation (instead of a separate tracing relation)
+- Grafana-agent will communicate to all principal charms it is related to the full list of currently enabled receiver protocols (not just those that a specific principal requested to be opened)
+
+## Benefits
+
+- Uniform tracing protocol/receiver story across substrates
+- Can enable tracing the same way you enable logging and metrics in machine charms
+- Uniform integration path for uncharmed applications
+- 'request one, get all' behaviour uniform with Tempo
+
+## Disadvantages
+
+- ??
+
+## Alternatives considered
+
+- Have different subsets of supported protocols in tempo and grafana-agent
+- Do not support uncharmed workloads on machine models, instead force them to CMR with tempo directly