fix(otel): preserve all exemplars per data point (#1662)

insert_exemplars wrote every exemplar of a data point to the same static
columns (exemplar_time_unix_nano, exemplar_span_id, exemplar_trace_id,
exemplar_value), so each exemplar overwrote the previous one and only the
last survived. This affected Sum, Histogram, ExponentialHistogram (and
Gauge) metric types.

Serialize all exemplars into a single nested `exemplars` array column
(array of objects), the OTel-faithful representation recommended in the
issue. Each element carries exemplar_time_unix_nano, exemplar_span_id,
exemplar_trace_id, exemplar_value, and any filtered attributes.

Add the P_OTEL_FLATTEN_EXEMPLARS flag (default false) to opt back into
the legacy flat columns for backward compatibility.

Also stops exemplar attributes from leaking into the physical-series
hash: `exemplars` is added to OTEL_METRICS_KNOWN_FIELD_LIST so exemplar
contents no longer fragment series identity.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: prabhaks

src/cli.rs

@@ -292,6 +292,14 @@ pub struct Options {
     )]
     pub metrics_endpoint_auth: bool,
 
+    #[arg(
+        long,
+        env = "P_OTEL_FLATTEN_EXEMPLARS",
+        default_value = "false",
+        help = "Use the legacy flat per-exemplar columns (exemplar_time_unix_nano, exemplar_span_id, exemplar_trace_id, exemplar_value) for OTEL metrics. These columns keep only the last exemplar of each data point. When false (default), all exemplars are stored under a single nested `exemplars` array column."
+    )]
+    pub otel_flatten_exemplars: bool,
+
     // TLS/Security
     #[arg(
         long,